Welcome to the site, Wayne!
Both of your questions have multiple variables that go into the equation to get your answer. As far as cost goes, how much you spend depends on how many parts you buy, vs what you have/find. I did not track my expenses as i built, i wish now i would have. It seemed to my that while the actual WK gasifier and and associated “big stuff” (filter and exchanger) did not cast me much, (except for buying my cooling rails new). it is the little stuff that adds up. (Red rtv, choke cables, welding rod, pvc, little pipe fittings, valves, rubber couplings, etc)
As Far as your second question goes, what would the purpose of your gasified vehicle be? Just driving around, or are you going to haul heavy stuff and pull trailers? One popular answer here for your “which vehicle” question, is what do you have? '92 '95/'96 dodge dakota’s are a popular choice for those who just want to drive here and there. For work, Wayne Keith drives a V10 ram. (Every day, I might add). Members here have gasified tractors, a cadillac, trucks of many make and models.
A third question that should be asked for clarity’s sake is which way of making woodgas are you interested in? Do you want to burn charcoal, or do you want to burn unprepared (except for chunking and drying) wood? Do you have an abundance of wood? What i wrote above applies to making woodgas with a Wayne Keith Gasifier. If you want to burn raw wood in a system good enough for daily use, the Kieth gasifier is the way to go. $50 gets you the book and 6 months of subscription to the premium side of this site. (Informational gold mine) $200 gets you lifetime membership. Both of those options are a really, really good deal. Great people here too.
Hope that helps, and may you have a blessed day,