There are a number of things you can have Dexters tested for. Not all of them would be meaningful to you.
Dwarfism (chondrodysplasia), color, dun, genotyping, parentage verification, A2 milk, PHA.
PHA is a "water baby", an embryo that doesn't develop lungs and accumulates fluids until it is too big to pass through the birth canal. Bad stuff. It is found in a single line of descendency in Dexters. You can test for it or research pedigrees. Testing costs more, but is easier and is accurate. I'd suggest that you get this test done and avoid buying positives. Make sure the test is done for Dexter PHA, which is slightly different from other breeds form of PHA.
Chondrodysplasia is the inherited trait that makes the short legged Dexters short legged. A calf that inherits two chondro genes (one from each of it's parents) is either born dead or aborted early. One gene is OK, two means a dead calf. I'd recommend testing for it, so you can make an educated decision.
A2 beta casein testing is available to determine if the cow or bull carries the genetics to produce A1 beta casein in the milk. A1 beta casein has been linked to health problems and a lot of people are trying to avoid A1 milk. This is not a Dexter thing. It's in all breeds. Read up on "A2 Milk" and make your own decision.
Color and dun probably aren't worth testing for, from how you describe your intended use.
You can get PHA testing from 3 different labs: Agrigenomics, Pfizer or Igenity. It costs about $25-$27.
You can get all the other tests from the Veterinary Genetics Laboratory at University of California, Davis. Or you can get them from the same source if you're testing registered Dexters through:
http://www.legacydextercattleregistry.com/info.php
The prices are lower through them.
Registered Dexters generally sell for a bit more than unregistered. The benefit is being able to trace their ancestry to find out which ones come from a line that has the characteristics you want. It could come into play when you try to sell your calves.
There is no way to register any Dexter that didn't come from two registered Dexter parents, so using a bull that is not fullblooded Dexter renders the calves unregisterable.
Dexters are a dual purpose breed, beef and milk, but some have been bred to favor one type or the other. The beefiest ones may not provide as much milk as you like and the milkiest ones may not yield as much beef as you'd like. There are still plenty of those that can deliver both. You just have to search for them.
Dexter temperament is the easiest going of all cattle. However, each one has it's own personality and has been handled different amounts. It would pay you to visit and inspect any cow or bull you're considering to make sure that you are compatible. That's never more true than when you intend to milk the cow. Once or twice a day under a cranky cow gets old in a hurry.
Modern Angus have a reputation for being harder to handle than your parents' Angus, but I've seen some that were still sweet natured. It's hard to predict how the calf will behave in a cross breeding. The calves I get from my gentle Dexter bull all inherit his sweet disposition.
People who show Dexters put halters on them at a very young age. I never put halters on mine, but I only have a few and each one gets a lot of attention and training.
Dexters don't need to be barned at night in most parts of the country. They need a shelter, but it can be fairly rudimentary and is best if it's open and airy.
I never bottle feed my calves. I let mama raise them until they're at least 4 months old before I even begin to handle them. This lets them learn how to be a cow (or bull). I'm there, feeding and caring for them, and the calves are used to me. They come to me when they're ready and I hook them on sweet feed and treats. They're very easy to train then and they'll always remember that they're a cow. The worst thing is for a cow to grow up thinking that you and she are buddies, for her games are too rough for you and you can get hurt. It's better for her to respect you as the Alpha member of the herd.
Dexters are a naturally horned breed. I keep the horned ones to preserve them as they were when I fell in love with them. They're very good with their horns and behave quite well around me. Here's a video of me working among the horned Dexters. They're my pets.
http://www.youtube.com/user/belindabowen#p/u/25/fyHiHaOceEM
Genebo
Paradise Farm