Regarding internal tonearm resonances, the MB-18 is likely better than the usual entry-level turntables of the time, that is, more rigid and more dead. The complaint we had at the time was that we could feel some slop in the tonearm bearings- no worse than other entry-level TTs, but not as tight as the previous S-arm version (MB-14 and 15).
To avoid aggravating that slop, we would use a high-compliance cartridge in it. The idea was those cartridges would send less vibration into an arm. Fortunately, the arm on the MB-18 was lower in mass than their previous tonearm, which allowed the high-compliance cartridges to work properly on warped LPs.
The rumble could be heard, but not until one played the music quite loudly on a good record- but the Micros were always better in this regard than the other entry TTs. Its level of rumble, though, precludes putting on a better tonearm.
What we found at the time is a) its level of rumble was low enough most folks were happy, and b) its low-friction and low-mass tonearm allowed the use of a decent phono cartridge so one's LPs did not get mistracked.
At your purchase price, you will someday be able to re-sell it with no loss to then obtain a fancier belt-drive by Micro, such as a BL-51 or BL-91 with or w/o tonearm, or one of their DD models, such as a DD-33 or DD-40 which came with their great MA-505 arm.
I would not recommend their DDX-1000 model. When we first sold them (and I owned one, too) their platters could be felt to wobble under one's touch, and we never did get great low-bass from it. This was likely fixed in its replacement, the DQX-1000, but I am speculating as I never got to use that one. I would up with a BL-91 and MA-505II arm, and am still pleased with that combo although I own others.
I will add that using an 'arc protractor' for your cartridge's alignment makes a lot better sound than any other method of cartridge alignment. There are methods to produce one on your own printer, but I do not know the links for that on this site. Perhaps that's found on the Vinyl Engine, and would require one to enter the tonearm's length and offset (I think that's all).
Also, we really liked the sonic improvement of a platter mat called 'Platter Matter'. There were other brands with similar names, but we felt this make was the best. It still comes up used once in awhile here, eBay and on Canuck Aadio Mart for $35 to $65.
Best regards,
Roy