Hi, and welcome to the forum. I'll try tp address some of your questions here, but there are other people on the forum who can give you better answers.
Since your budget is fairly tight and you want reliability (who doesn't?) you should, in my view, dismiss both the later wtater-cooled buses (too expensive still for a good one) and also the early "splirt-window" kombis (these deserve a bigger budget to own and maintain, plus parts can be hard to find). This leaves the '68 to '79 "Bay-window" buses.
The "Bays" are relatively plentiful, spares are widely available and, when properly fettled, can be quite reliable. The earlier examples of these, especially the 1968 to 1972 models, have drum brakes and an engine essentially the same as the Beetle, a 1600 cc boxer.
The '72 to '79 models have (1700- rarely) 1800 or 2000 engines, with the latest models being fuel injected, and disk brakes. These engines are heavier-duty than the 1600, being more suitable for round-oz travel, but are substantially more expensive to recondition.
If your mate has some experience in VW
motors, looking for one which needs some attention may be a good plan.
To see a summary of the Kombis which are available at this site, visit:
http://www.aussiekombicampers.com/
There, under the heading Kombi News, you'll find all that are currently for sale here.
Good luck!