You can put a latex over just about anything. The reason for an oil-based primer is to prevent bleedthrough of darker colors, stains, etc. And, in an old house with the possibility of lead paint, this will usually meet standards for neutralizing/encapsulating it. No need for expensive testing if you just go ahead and do this in the first place... So would actually save you money in some minor degree.
When I had my painting service (take into account this was in burbs of Chicago) my rate was: $25 an hour, customer supplied paint. Sometimes I would quote a job based on rooms, cause I'm pretty fast, lol. Then, for an average living room of 12 x 24 would be $300. Would also consider barter, though, which lots of folks liked - too much for comfort, lol!
The price goes up the more woodwork and cutting in that's needed... In older homes, there can be a lot of decorative stuff which might require hand work.
Everything round here is extra... If you want walls washed, same rate as painting. Spot cleaning $25 an hour. etc etc etc. If the job required scaffolding (cathedral ceilings etc) rate was $35 an hour (but I rented the scaffolds).
I was also bonded for $1 million.... Would be a good idea to check. Covers not only theft, but also damage from accidents. Insurance is also expensive, any time you go into someone else's home, you are considered as a threat to that person - and their property - by the insurance companies, lol.
I also requested homeowner help move really heavy furniture (usually no problem, I'm pretty small!).
I had no lack of jobs.... even was hired by a couple gc's. Problem was wall paper. Don't mind hanging but HATE removing, lol!
Sue