OK Mark. Some RTRs have made it thru the test of time, while others not so well. It seems that most decks that were belt driven or had lots of belts, where obviously troubled with worn/broken or stretched out belts. Tape head wear is another issue as well as the rubber pinch roller.
Decks that I think stand the best chance at being usable in this day an age are: ReVox, Technics direct drives, some Teacs, TASCAM, Tandbergs, and perhaps some Sony.
Marantz did make a couple models if memory serves me, but, like their cassette decks, didn't fare well.
If I were to get one, it would be either one of the 3 I listed, ReVox, Technics DD, or a Tascam.
Now there are 1/4 tracks and 1/2 tracks, some that are 3 3/4-7 1/2ips, some 7 1/2-15ips, some 15 ips.
Some with built in Dolby B, others without. 1 mil tape is recommended.
With a 1/4 track, you can record in stereo and get 1 album on each side of the tape. The tape heads usually use tracks 1 and 3 for one direction and 2 and 4 for the other. So, you can play one side thru, flip the tape around and play the other side.
1/2 tracks are only 2 tracks on the tape, so you can only get one LP on the tape. If you were to flip the tape around, the music would be backwards.
Hence, on the LP Face the Music, by Electric Light Orchestra, side 1, first song (Fire on High), you will hear backwards recorded spoken words. With a half track, simply flip the tape around and hear: "The music is reversable, but time is not, turn back, turn back, turn baaaaaaaaaaaaaaack", which is way cool.
Most audiophiles record at 7 1/2 ips, so on a 7" reel of tape, you will have about 45 minutes of recording time per side (one LP). If your machine is 3 3/4 ips, you can get 2 LPs per side, but the tape hiss will increase.
At 15 ips, you will get one side of an LP on it (7"), and usually those decks have the capacity of 10 1/2" reels (which you can get the full LP on one side).
You can still get new tape, but it's spendy.
As the other poster commented, are you sure you want to get into this?
Wayner