I've had my Akai 4000DS since student days, late 70's, so have a serious amount of stuff on reel to reel, John Peel, my mates bands and late Fathers Jazz band, brothers band, all recorded at gigs in the 80's and 90's. Now, the old Akai is quite tired, it still works, but clutches etc are well tired. The heads on them seem to go on for ever however, but there are issues with certain transistors and ic chips which can make these machines , er "troublesome". Check out the "Tapeheads " forum, or indeed the UK based Vintage Radio Repair forum. I bought, after a lot of careful research and dismissing a lot of less than "kosher" used Revox A77 and B77 machines, a very late A77 mk4, WITH GOOD HEADS!!!!!. Just about everything else is an easy sort out. Beware faulty VU meters, expensive to replace and very hard to fix. I always wanted a Revox, or a Ferrograph Logic 7! I also have a quite rare Uher portable, a 4400 stereo. Bought as a non runner, didnt neeed a lot to sort, again good heads!
The good thing about the Revox A77 is, at least they are a relatively easy machine to sort, lots of parts out there and people who are knowledgable about them. They also sound good!
For a good all round rugged "semi-professional" machine, I would suggest a nice sorted A77. The smaller 7" jap machines were very much a domestic machine, usually single motor, with belts and clutches, which wear and give trouble. The bigger Akai, Sony and Technics 3 motor decks are very nice, but getting silly price-wise now, with little in the way of spares. I would avoid anything valve, old mono only stuff like Brennel and Ferrograph etc. Stereo Ferrographs like the series 7 and logic 7 were good machines,in their day, but have not aged well, perished rubber bits a particular problem!