There are many dual purpose breeds out there that could fill your needs as well and possibly even better than Dexters. I have a few concerns that Dexters are being blown out of proportion in their ability to produce etc, because of publicity and more demand for them. I've heard some very ridiculous claims by breeders and one should really do their research and buy from a reputable breeder to be sure you end up with what you want. Not saying that you are this way (going with the big hype that seems to be going around with Dexters right now), just mentioning some concerns that many have.
Red poll, milking shorthorn, belted galloway etc are a few larger breeds that would be good dual purpose animals. They would cost less than a Dexter to buy and would produce more, though they would also eat more.
It depends on how much milk you want, how much meat you want, how much hassle you want (milking and caring for 2 animals to get the same milk you would get from one Jersey), what your climate is like, your experience with cattle etc.
Dexters are not commonly milked, so it would be up to you to train them. For a newbie it can be an interesting experience to halter break an adult cow and then train them to milking, while training themselves how to milk. Especially hard if you take their calf away and they don't want to let down and are just being stubborn and grouchy as they haven't been handled before.
They do NOT give 4 gallons of milk a day unless you pay top money for a proven cow with phenomenal breeding and bloodlines. The most I ever got was 3 gallons and usually closer to 2. If a calf is getting half of that, then you are only bringing in a gallon a day which is not a lot for butter, cheese, icecream, yogurt and drinking, especially if you have a large family. Then you need two which brings up the cost (buying two animals which are usually minimum of $800 each for a good animal), the workload (milking and caring for two animals),butchering costs for 2 steers, and the feed. You may find it becomes a burden to wash, train and milk two animals when a quiet already trained Jersey would provide lots of milk with less work and less start up cost.
They have not had the extensive breeding that goes into a dairy breed to bring up their milk supply, breed better udders etc. So you have to be pretty particular when buying, to be sure you end up with the temperament and conformation you need for a good dual purpose breed.The gene pool is not as large and because of their size you can't just take them over to the neighbor's bull to be bred. Instead you either need to own your own bull (which can be expensive if you replace him every few years), or pay for AI which can also be expensive if your cow isn't catching.
Being short makes them harder to milk, especially if they have a larger udder. You can barely get a bucket under their udder. THey have very hairy udders which make milking an adventure while pulling hairs left and right. But the hairy udders protect them from frostbite and other udder maladies in the winter.
They kind of shuffle along on their heels (not picking up their feet like a normal cow), which ends up causing their toes to grow long and not be naturally trimmed and need more care in that respect. Might not be a great deal to you, but thought I'd mention it.
Having said all this, I have owned Dexters going on 4 years now and love every one of them (I have 15). I am a young single woman who has to do all the work by myself, and I could not handle castrating, dehorning, halter breaking etc on the larger animals, and yet could rope an adult Dexter and work with her/him. THe bulls are very laid back (but still bulls!), and that was a major plus working on them.
THey make wonderful mothers and calving problems are very rare (but they can be rare in many other breeds).
I have had great success with carcass weight, but that is not the general rule. If you have a need for huge amounts of meat, one 18 month old steer won't provide more than 2-300lbs of meat, but it is very good quality and delicious meat at that!
Mine are very healthy and I haven't had problems yet.
We get very harsh winters and for someone wanting a low maintenance animal you can get a bit of milk from, Dexters are great. They can handle our -40C weather with just some trees for shelter, whereas the dairy only animals are not meant for that kind of treatment and will freeze teats and loose weight (not to mention shivering because they have no body fat!

. But there are other dual purpose animals that can handle this kind of weather also and comparing them to a highly bred dairy animal is not doing the dairy cow justice - just a warning. :<) I still have one Jersey, but in freezing cold weather, mucking out stalls (because they can't be outside in colder than -15 to -20C), pumping in tons of grain and hay to keep them warm, etc gets tiresome and costly. I have calculated down to minerals, vet costs, slaughter fees, meat from calves etc comparing Dexters and JErseys and I can keep two Dexter cows on what it costs me to keep one Jersey. I will get 2x the amount of meat from two Dexter calves, but I won't get the same milk production from 2 Dexters that I will get from one well bred JErsey (I have one Jersey I was getting 8 gallons a day from, 6 gallons being a bit more common/standard for your backyard Jersey). But I am not into high maintenance, and I have found my Jerseys take a lot more fiddling to keep them healthy (barn, heat etc), they have had more healthy problems than any of my Dexters, and are not as good at producing on grass alone like my Dexters are. My desires were to have healthy, thrifty, low maintenance, grass raised animals who calve easily, are fairly quiet and easy to handle, and give me enough milk to drink. The Dexters easily fill this and my Jerseys (while having their place when you need more milk) did not fill even one of them. The milk production I didn't need was high, and the rest was low, so I had to decide what was more important to me.
But I do love the Jersey personality and the fact that I can milk for only 3 months and get a years supply of butter and cheese for a family of 7 adults. One reason I still have one is I'm trying for a Dexter x Jersey heifer - if only they would stop throwing bull calves. ;<) But as I go out in the morning and see my Jersey get up stiffly from the cold weather seeping into her too unprotected joints (My Dexters are hairy critters!), see her teats grow chapped and worry about frostbite, watch her consume a full bale of hay and 6 lbs of grain just to keep her healthy (and still showing too many ribs for my liking:<), see the pile of manure in the stall that needs to be mucked out daily (which freezes solid in our cold weather), I realize that this climate is hard on the sweetheart dairy cows that were bred to be kept with a herd of 30 others in a warm dairy barn.
I may have just muddled everything in your mind, but just some observations I hope will be somewhat helpful.
Heather