Hi Drop Out,
I agree Skoolies are better than RVs, construction standards do not compare. The steel on steel bus construction versus the stick and staple of RV construction. Plus, you nailed it knowing everything about your future home is critical when you build your own custom home on wheels.
As far as the budget, I am not sure. A lot of folks here have put a lot of money into their skoolies and had amazing end products and a lot of people are very good reclaiming, repurposing and salvaging materials with amazing results. It sounds like time is on your side, my understanding personally and with observation is the more time the better.
Your research and planning is critical and you seem to be above par on that front. I am moving in the same direction as you, a mix of power sourcing, solar, shore power, self generated. My regional climate is extreme hot humid summers and bitterly cold winters. AC and Heat is important, the power sources are of more importance.
Be prepared for constant changes in your lay out design. My lay out was perfect several times and slightly changed several times. You can do it and it will be fun and worth it in the end.
D.L. Jones III