Some of that may depend on the state you live in. As aforementioned, the 76/77 yrs had factory disc brakes up front and power steering and power brakes were options from the factory, but were not standard equipment-as some think-just pretty common options. The Big Bearing rear end was also factory equipment in 76/77 and are slightly more desirable than other 9", but really ANY 9" rear end is better than no 9" rear end. But otherwise, as mentioned, after mid-71 the D44 replaced the D30. The D44 is much more desirable than the D30. The drum brake versions of the D44 (71.5-75) can easily be converted to disc brakes using either readily available Chevy or Ford parts. You can do a search using hte function above and spend hours going through all the info...probably also in the tech section too. The 66-68 broncos had removable door frames, which some find desirable. But really any Bronco is or can be built to the level of any other Bronco pretty readily. Like I mentioned before...read, read, research and find some local folks to get a little "hands on" time.
Building one (up) or rehabbing one is a great time and a great learning experience to become acquainted with the truck...however, it is also probably the more expensive proposition...the most expensive option is having someone else do the work for you; can you say money-pit? You will have some guys say, "OH, thers no way that truck is worth that much $!" Sometimes, this is true. However, the market determines an items value and Broncos are the shit...DUH! Most of the time, buying a very clean, refurbished, rebuilt, restored, or resto-modded truck is a cost-effective option....most of the time the seller is loosing money...unfortunately. Notice how I used the different adjectives above; thats because as with most used vehicles, there are varying degrees of "rebuilt/restored, etc"...make sure you know what yours is. Buying a truck with a solid body, I would argue, is often a better trade off than buying a mechanically sound truck that needs alot of rust repair or body work. The mechanicals are the easy part...and probably cheaper too. You also need to know where to look for the "problem spots" and rust. You can do a search on this too. Dont be shy the search function works very well on this site and members are not afraid to give a plethora of opinions, if you couldnt tell. Really restoring or resto-modding a truck--with you doing the work--in stock form will probably cost about $20K, with upgraded components, etc its closer to $30K and up, up, up. So, if you find one 'mostly' finished with upgrades in the 15-20K range you will come out ahead, if the specs suit you. Oh yeah, since you are new; they are NEVER really 'finished'. Best of luck on your vehicle-quest.
Just as a ball park figure; a driveable starter Bronco should be obtainable in the 3500-6K range, depending on condition. Anything less, should probably be thought of as a steal (if its in good to moderate condition) or a project/parts/starter vehicle. Anything above $7500 should be pretty clean, straight, modified, lifted, or built or some combination of such and the price goes from there. Again, good luck.
John