We have a classic 310 motorhome (31-foot) and LOVE LOVE LOVE it!
The first thing I will tell you is it is very easy to drive. The second thing is that I do not think you will have enough room with the two kids in a 26-footer. It does not have a lot of storage space and you will feel cramped. It would probably be okay for occasional weekend trips, but for full-timing you will want more space.
A slide would help with the living space and I have not heard anything about their being unbalanced. The issues I have heard about with slides is they leak, and they can have problems with the motors that make them move. The classic mohos did not have them, but some of the newer ones do.
The stuff you would be seeing a mechanic for is mostly truck-chassis stuff (RV-makers take a regular truck chassis, like the Chevy P30, and then stick an RV on top of it), so they will know it very well and have parts for it. The only thing you would have to be "scared" about is if they would rip you off on service/parts, but they would do the same thing if you were in a car (just maybe on a bigger scale since they might try to convince you that stuff costs more since it is a motor home). If you have some knowledge of the vehicle, you can hopefully alleviate that. And always ask to see a part they say needs to be replaced.
Regular mechanics will usually not work on the "house" part of the vehicle (plumbing, interior wiring, etc.). You will need to go to an RV shop for that. It is not quite like house stuff since it uses a lot of RV-specific stuff, so a home repairman is probably not a good idea unless he is familiar with RV work.
One thing about buying an older moho is that the price you pay is usually just the "price of admission." This means there are always things you will have to fix/replace before it is right (front suspension, shocks, dash a/c, roof a/c, heater, etc.). If you can find one at a dealer where they have done the work already, that may be the best way to go. Prices should be very negotiable.
I would think kids would travel a lot more easily in a moho than in a car--like you said, more room to stretch, plus it's a lot quicker when they have to go to the restroom to just pull over and have them run to the bathroom (instead of having to get into a trailer, etc.). You will be able to see the sights out of the motor home better than a tow vehicle (sits up higher), as well. The drawback is that unless you tow a car or something bhind you, you will have to use the moho for running errands or other driving trips. If you are parked full-time with a car, that works fine.
I hope this helps you! There are a lot of people on the forums who are willing to help with inspections if you have found one you like. There may be someone with moho experience in your area. Here is the topic
you can look at/post in for info.
I hope this helps! Good luck with your search!