Since it sounds as if you will be doing this conversion on a budget, don't go cheap on the bus.
The actual cost of converting the bus will be pretty much same regardless of which bus you actually purchase.
But if you get a bus with the amenities you would prefer (air brakes, big HP, big transmission, highway gearing, etc.) you will most likely spend less $$$$ in the long run.
Don't fixate on a particular bus. If it gets sold before you can finalize a deal I can guarantee you there will be another bus that will come along. Many times it will be in better condition, have better spe'c's, and most probably cost less.
One of the most important things not to purchase is a bus with rust. A little might be fixable but a lot will be a cancer that will continue eating away at your bus until it collapses in on itself.