Well, Bob, if you talk about heads, basic transport parts, conversion kits such as 1/2" to 1/4" or 1/4" to 1/2",
John of Sprague Magnetics or JRF can help you on both APR and MTR-15. Electronics wise, as far as you keep
firmware EPROM backups, minor repairs can be easily done. For other mechnical parts, part machines are
the only way to go. It's possible to get Otari parts directly from Japan yet the source has not been reliable.
Again, John may hunt for certain parts for you, and I have found out APR is easier than MTR-15.
Talk about MTR-15, it's the most sophisticated machine I have come across. Mechanical parts are not as good as Studers
yet the electronics are simply second to none. Very well assembled with lots of attenion paid to details.
The downside is the head amp is all OPs with lengthy signal paths, even worse than A820. This hurts the sound quality.
The heads are hard permalloy... great sounding yet dont last long...
Don't be scared off by my words on the MTR. It's more reliable than APR, newer than APR so it shall last longer.
You can have someone do OP swaps then it will sound superb!
In either case, if you can get a good deal, I'd say either APR or MTR-15 will be a great pick.