To transfer to a computer, you ought to be able to hook your audio outs direct to your A/D box and then into your computer. If you get a cassette unit that has auxiliary outs for each channel, then you can transfer all your tracks at the same time provided you have enough inputs on your A/D box. Otherwise you'll have to transfer a submix or else try to transfer your tracks separately in several passes. You could get some problems lining the tracks up again if your tape speed is inconsistent, but it can be done.
I got started recording on a cassette portastudio back in the 1990s, a Fostex 280. Man, I thought it was the shizzle! Prior to that, I was doing live overdubs while bouncing back and forth between two tape recorders. Still have the Fostex though I haven't checked the condition of the rubber parts in a long time. It used the high quality cassettes which were getting harder to find last I looked.