The 7.5 is a speed, not reel diameter. Most pre-recorded commercial tapes are 7 inch reels. In the early days of pre recorded the format was two track, stereo. It quickly changed to four track, stereo. Most four track were issued in either 3.75 or 7.5 speed (double 3.75). Quality of re-recorded commercial tapes is all over the 'quality lot'. The best are the Barclay-Crocker 7.5 tapes with Dolby b coding, and they are also considered the best since they were produced in a duplication process that was not high speed. I actually watch there process, in person. There are also 'broadcast' pre recorded tapes, which came on 10 reels, were two track stereo. These are what most broadcast stations used until cd's came on the scene. Quality was very high as Bill Drake was a nut about quality, they ran at 7.5 speed and were encoded with automation signals that let four reel be mounted and played continuously. Drake-Chenault is the brand most people know when it comes to these broadcast tapes, there is a web site up on its history. Also, and very hard to come by, were broadcast tapes made by the Armed Forces Radio and Television Service. These were 10 inch reel, four track, 3.75 speed. Each side had about 50 cuts and most of the auto reverse machines sensed the leaders to provide continuous play. These AFRTS tapes were very high quality, and they should have been seeing us tax payer's went for 'nothing but the best'.
I have about seven reel to reel machines, including Technics, Akai, Sony and Teac/Tascam. What one machine would be a good start? Akai GX-635D. As mentioned, Teac still does service and has parts. I use people that brought out Teac's service business on the East coast.
As you know, the downside of reel to reel is lack of machines in excellent shape. Just like turntables, pick up is a must. The UPS guy's love to crush these machines, original box or not.
Lastly, pre-recordered tapes are a crap shoot. Start slow.
