Good answer JR, you pretty much covered everything I would say.
Tasc, all this vintage gear is out of production, and IMO, your best source is Ebay.
The TSR-8 is also a Tascam 1/2" 8-track, that's a little newer and more modern than the 38, and has LEDs and built-in dbx.
The 38 is the more vintage 1/2" 8-track, with VU meters, and no dbx. The DX-4D's [2] are the add-on dbx units.
Also, it should go without mentioning, but the 38 will work perfectly well without the DX-4Ds, but with dbx, in this case as an add-on, it really minimizes tape hiss, and boosts S/N ratio dramatically.
Go surf Ebay/Tascam. With all the pics and descriptions on the listings, it turns out to be a pretty good resource for info.
Thx, & good luck.