Hey, don't get discouraged!
The Gillig LF40 is indeed a great bus, but not for the uninitiated. It is very heavy, and compared to a skoolie everything on it is complex and super expensive, from the multiplexed wiring harness and the kneeling air ride suspension to the Allison B500R transmission.
As it turns out, there's one of those El Dorado RE coaches on ebay out of Canada right now. Search for Item #392238664479. I think this is a 32 or 35 footer, and it has the ISC 8.3 motor, which is a gem. Beware, though, this bus has been for sale on and off for a year or more, never listed with enough pictures or description. If its a mess or hasn't been driven for five years its way overpriced. New ebay user with zero feedback. Also needs importing from Canada, which I know nothing about but could be a chore. Danger, Will Robinson, danger, danger! Of course it could be a super sweet bus and a perfectly good deal, too...
Relax and keep following your dream- when you're ready and know exactly what you want, the right bus will find you. In the meantime, work your floorplan out in the garage or the kitchen. Start a spreadsheet for project costs. Follow build threads on the buses you like to see the pros and cons. Not having a bus at the planning stage is actually a huge advantage. Become an expert in what you want out of your tiny home without the burden of already having a platform with all its needs and limitations. Go to events and tour converted buses. Tour new and used motor homes for ideas and an educated sense of what fits into where. Decide WHERE you're going to go and park, how much fuel economy you MUST have, and things you CAN'T live without on the rig. Much better to work out this three dimensional (five if you include cost and time) puzzle without having big pieces of it you may not use in your driveway and garage!
The more educating yourself, deciding, planning and designing you can do without lifting a finger to building the better. To that end, ironically, getting a bus should be the last thing you do...