Hello Antti,
I am afraid there is no way around spending some cash here, how much depends on you idea of sound.
There are several different modules made during the tube and transistor area, but only 10% are good as mic pres. About 10 to 15 years back radio stations all across europe dumped out the old stuff, mostly if you picked it up it was yours, today there is nearly nothing left and the old junk is getting traded high on e-bay etc.
There are not to many good useable mic worthy units, so most dealers sell everything that bares the name Telefunken, Siemens, Neumann and TAB as very rare something. People contacted me if I could help them to get their U370 compressors running, they paid about $700 for a pair and were told a very special version of the U373... By the way the U370 is a light meter driver....
Routing, buffer and everything that gives you some sort of gain is always labeled as the classic Tele sound, etc....
Without the proper interfacing units or buffers, there is not to much what you can use, mic pres were either V72,76,77 and 78, forget about the a/b/t stuff they are just buffers and may or may not sound good as mic pres...
The transistor area units were based on the tube modules so designations are the same V276 (376, 476 and 676) are designated mic pres, and the V272,372,672 are mic pres but need in most cases modifications to sound good.
Everything else ANT, Lawo, etc. is not worth the price because they do not sound the same than the tube and real transistor units.
I do not think API, Neve etc. compare to the german transistor modules, entirely different battle class....
To rack or not to, is quite a difficult question, first I would put the money were it is badly needed, rebuild the units first, make sure it is done by a professional and done to specs, before you get into the cosmetic part.
Racking, very nice to have but most racks are quite questionable for the price they are offered. If you buy plug and play racked units, make sure it was done right...
To answer your question in short, there is no shortcut into good sound, if you want to V76, save the money and buy one, if you do not want to spent get something that is cheap, there are several hundred mic pres out there you can get for under $100...
If you have lots of time check out the Surplus offering of german radio stations.
Here is one link:
http://www.wdr.de/unternehmen/t-boerse/index.jhtmlThere are about 20 government owned in germany so lot of potential to get some cool stuff...
Best regards,