I have 3x 3000NL's--it's one of my favorite computers since it was the first PC I ever had. And holy corrosion, yours got hit bad. Sounds like you're having a rougher time than I did using it. I haven't had to do anything to mine so far--the daily driver I'm using just got a good cleaning, but runs like a champ on all the original caps and everything.

So some feedback to the issues you're having...

The 3000NL powers its 1.44 floppy over the ribbon cable, so that may be why you're having odd behavior with the new drives (if they're normal drives). You have 2 options, either get replacement Tandy drives, or you can mod the cable or new drive pins to remove the power being received from the ribbon. I put a Gotek in mine and actually ended up just clipping a bunch of pins and it worked fine. See this VCFED link for details.

On the RAM. If you don't have the 8-slot extended memory card, to get 640k you have to populate the 2 SIMM slots on the motherboard, AND you have to add 128k of DIP memory in the sockets next to it. The proper procedure if you want more memory than that is to remove all memory from the motherboad, and just populate it on the expansion board--i.e. do something like 4x256k SIMMs for 1MB. Then the jumpers must be set accordingly. If you need jumper settings, you can check the faxbacks on Oldskool.org, but it sounds like you may have that info already.

Finally, I have an original print of the 3000NL service manual. If you still need it, I'd be happy to do some scans of whatever you may need out of it.