Hi there
I guess you looking for a multi-track machine,
Tascam 22-4 also have sync-function between all 4 channels.
Speed: 7-1/2 and 15. Very dependable deck but only 7 inch's reels.
RCA-in and out + DBX connection.
Most parts can still be bought new from Teac
If you want sound on sound fucntion, why don't find a Teac AX-10 Sound on Sound & stereo Echo unit. This you can connect to 2 channels on any deck with RCA in and out.
Another good multi-track machine is Tascam 44 OB, (4-track)my personal favorite, has more bells and whistles + the best 1/4 tape sounding machine I have heard.
2 speed rewind/fforward function, pitch-control, direct-drive capstan-motor and dependable. Both RCA + XLR connections + DBX if you want to add on later.
Link with photos and spec's:
http://www.angelfire.com/electronic2/vintagetx/TASCAM44.html
I have used both 3340 S and 3440, slight difference between the sound, more sound-spectrum on 3440, deeper bass.
The 3440 is a popular model and built like a tank.
Look out for worn heads as that is common for a well used machine.
3340 S is cheaper, they were made in 2 different versions, the early one has a slide-switch for the transport-section, I would stay clear from that model, have seen alot of transport problems with failing relays and burned out switches.
3340 S can be found pretty easy, look for a low hour machine and if you can test it first, that's always a plus.
Here is a link with photos and spec's for 3340:
http://www.angelfire.com/electronic2/vintagetx/TeacA-3340S.html
Feel free to ask any questions,