It's okay, but it wasn't/isn't top of the line.
One of your main questions is about the speed. It's a double speed recorder, which gives higher fidelity, but it's not compatible with single speed recordings from another Porta or home stereo/commercial cassettes. This is why someone else might have posted about it playing "too fast". That's a feature, not a problem, but be aware of this compatiblility issue.
It has 4 mixer channels, fixed hi/low EQ, dbx NR, pitch control, zero-stop and LED meters. 2 of the input channels have mic preamps, and the other 2 are unamplified, straight line inputs,... all 4 inputs on 1/4" jacks. It records to cassette tape up to 2-tracks simultaneously, max.
You may take your old double-speed/dbx Portastudio 4-track tapes and mix them down to stereo (& output to the computer in stereo, if you wish), but it does not have 4 tape (track) out jacks. It will not output 4 discrete tracks simultaneously, if that was your intent, (a 4x4 dub to DAW/computer interface). Your outputs will always be mixed to stereo, to the outside world.
Sound quality should be acceptably good. If you want something that's relatively nice and can double as a home stereo deck, I might suggest a 424mkII or the 424mkIII. If you can find a 246, it's a real sweet setup. 464 would work. Probably some others I'm not mentioning, but you want one that records at High and Low speed/switchable and dbx On/Off (switchable).
Verify if your tapes are single or double speed & dbx,... & if they're single (normal) speed source tapes, you'd be alright with a Porta One, Porta Two and some others.
What 4-track did you record your legacy tapes on? It makes a difference. Fostex (DolbyB) tapes will not be 100% compatible with Tascam (dbx) tapes. If they have no NR at all, your choices change again. A Porta 02mkII is Normal speed & has no NR, but higher models mostly have dbx on/off switches, which is a handy feature.
Sorry for a long response to a simple question. All 4-trackers are not created the same. The more features you have, the more flexibility you'll get.
Please see this page for a long series of Porta 07 photos.
Pictures and images Tascam Porta 07 - Audiofanzine