1) The Denon is from 1974. Be careful about spending a lot of money on anything that old, unless you can fix it. Denon frequently used a magnetic servo (tape head) feedback circuit to maintain speed accuracy, and some of their tonearms used electro-mechanical resonance damping motors. My guess is that those mechanisms are unfixable by anyone, today.
2) That said, DD record players are generally known for longevity. I have an SL-1100a from 1975 that runs like new.
3) That said, I have a Garrard Z-100 record player from 1972 that still runs like new. However my Dual 704 from 1977 recently gave up the ghost. Once dead it's typically trash bin time for old machines. A Dual electronic DD could possibly be fixed, but you have to ask yourself whether it is worth your (or someone else's) time and money?
4) Denons generally had no suspension (or really any acoustic isolation to speak of). Denon accessory solid wood bases were quite lightweight and resonant. Looked nice but didn't provide for much insulation from external vibration. Upper level Denon integrated (arm and turntable) players usually featured a laminated faux rosewood glossy finish that looked fantastic, but was actually only for looks. Those bases were often simply hollow frames, or pressed particle board sitting on a plastic foot with felt or rubber on the bottom.
5) SL-1200 Mk variants are pretty well isolated from acoustic feedback (using a lot of mass and rubber damping). Unfortunately, as with most things, inflation and cost cutting are the norm. In 2005 I paid $500.00 for my (made in Japan) Mk5. The current Mk7 (made in Malaysia) is one thousand dollars. It would surprise me if the new ones are built as well as the previous Mk models, but I don't have first hand experience with the newer ones.
Of course the current 'audiophile version' (Japanese made) 1200 sells for four thousand dollars. That model is going to have the best fit and finish, is the one that will no doubt provide the best acoustic isolation, and the one most likely to last another fifty years (or longer).
6) 78 rpm is a nice addition (and necessary if you have 78s in your collection). Otherwise it makes using a Discwasher brush quicker, but that is about it.
7) If you don't need pitch control (I've never used mine) the 1500C appears to be decent value, considering what is out there. You get a phono stage and an Ortofon cartridge, which may or may not be important to you. For a budget oriented purchase, I would buy one of those (or possibly the Mk7) over a 50 year old Denon record player. If I wanted total quality, the $4000.00 model would be my choice.