Steve, I'm going to try and address everything but I am much better (and more patient) having a conversation in person rather than typing (and I suck at typing.....). As far as the costs go, I think those numbers are doable (although I think that a 30-40 year old school bus purchased for $4000 is going to need some pretty serious maintanence pretty quick-tires, bearings, brakes, etc.) but doable isn't what you are after I think. The reason for wanting to build from scratch is because you want to do it right the first time. As someone who is constantly undertaking projects for 2 reasons (what I want isn't made or it is made and I can't afford it) I can tell you that things always snowball. It's the nickel and dime things that really add up over the whole project. I usually make a spreadsheet of all materials before a project and if I stick exactly to the design I usually spend pretty close to what I calculated, however, with many projects the design changes while you are building (usually for the better) and it is not uncommon for me to spend 50% or more over budget on a remodel or other project.
As as far as a school bus being built better than a regular rv, I'm not so sure about that. As long as you only compare to diesel pushers (built on a bus chassis usually) and super c's (built on heavy duty truck chassis) the reliability of the vehicle will be comparable. The interior may be less so but what I have seen of the bigger RVs they are not so concerned about weight like the smaller ones so the living space is constructed fairly well. What I think you will find is that most RVs of that size are likely newer than the school bus and will have newer tech (engine, etc. that have easy to find parts) and the RV is more likely to have had recent maintanence (aka the bus may have been parked out in the field for the last 10 years while the RV will have -most likely- been taken care of slightly better). The repairs would be more along the lines of vehicle maintanence, except that instead of parking the broken down vehicle at your house and working on it as time/money allow you have to get it fixed quick because it
is your house. I'm sure you have already read all about the costs and maintanence/break downs from technomadia but if not here is a good post with their thoughts and costs:
http://www.technomadia.com/2014/06/three-years-in-a-vintage-bus/
The search for the perfect vehicle is a long one for sure. It is tempting to just build something because then you get exactly what you want. The problem is you want to live in it ASAP. This causes several problems. First is you will be rushed/pressured to get it done soon. You won't have time to get everything just the way you want or if you take the time you will likely get burned out and sick of the bus by the time you are living in it. Second, you haven't lived full time in an RV so invariably after it is "done" and you have been living in it for a while there will be things you will want to change. Some of these will be minor easy changes but some of them will likely require a bunch of rework. Maybe you will even decide that you want to start from scratch with a different platform (perhaps one with underbody storage, more/less weight, power, etc.). This will be tough because the resale will be low. Floral print wallpaper and many other things in a RV is superficial and fairly easy to change. I think your best option is buy one that will "work" for your needs and can possibly be remodeled to be better. Maybe a little up front and then a little at a time while you are living in it. This spreads the costs and the man hours around as well as allowing you to get to know your needs a little better. Then, if a few years down the road you decide to go a different direction (different platform, different lifestyle, whatever) you at least have something with a decent resale to help you get started. I think that 1993 RV would be a good option (I didn't look at it super close) because it's ready to go, could be modified, it's in good shape and not a ton of money. Why don't you like the idea of keeping the bikes in the toad? A small pickup like my Nissan hardbody or a first gen Tacoma with a shell on the back could hold the bikes and some other recreational gear quite well I would think.
Once again, I don't think that building from scratch is a bad idea-under the right circumstances. For me those circumstances would be several years (possibly leading up to retirement) to build and test it with no deadlines and opportunities to research and test individual parts and pieces to come up with the best result. I also don't love the school bus platform for the reasons listed here and in my last post. However, if you decide to build from scratch I would love to help however I can. I usually think helping with other people's projects is fun because it's their money, their deadlines and their stress.
If if you are still thinking seriously about building, start by working up a realistic cost spreadsheet. Begin with the components. Fridge, micro, TV, heater, water heater, pump, composting toilet, sinks, water tank, gray tank,
batteries, inverter/charger, solar, furniture, A/C, etc. Building material will be a little harder to estimate unless you have a detailed plan but there are probably some online resources for estimating remodel costs by the square foot that could be used for a baseline. Then look realistically at bus purchase cost and what it would need to be made road worthy (dry rotted tires, old seals, etc.). Then account for odds and ends and a bunch of stuff that got forgotten and that should get you a
rough estimate for the build cost. Good luck with the decision and search. Call/text me if you want to discuss in person. Pm if you don't still have my number.