Re: lookin to buy a horse!!
Most Bronco's I have encountered have been modified slightly from original. You may want to find one that someone else has dumped their life savings into.
71 was the first year for the Dana 44. 74 was the first year for the calatytic converter and power steering. You may want something in between. The first upgrade you will need to look for is power steering. 31 spline axles are also nice to have. Some broncos have 28 spline and 2 spider gears which aren't nearly as strong. You may want to upgrade to power brakes, Disk brakes, etc. 76 and 77 had disk brakes.
I bought my 71 and since then have changed gear ratio, new NP435 trans, Power Steering, a lift kit, and a few other minor mods. I still have the drum brakes and no power brakes though.
There are 2 different 3-speed manual transmissions to look for. Tag numbers will either say RAN or RAT. Stay away from the RAT, It is an inferior transmission. If you are looking for an auto trans, they come with a C-4 in 74'. I have seen some modified broncos with C-6's. A C-4 will work fine with a slightly modified 302.
As for the engine, either a 289 or a 302 will do. You could swap in a 351 windsor later, but you will have trouble fitting it under the hood without minor modifications. The 302 is much more efficient and will push the pony just fine.
The in-line shifting Dana 20 transfer case is indestructable! There is a "U" shift Dana 20 transfer case that has problems slipping out of gear.
Hope this helps,