I would recommend the 3650 + 300DC combination as offering a lot of flexibility and a really nice, rich sound. Both of these are comparatively rare, though, and may be expensive. (I was lucky enough to find several of each at well-below-market prices a few years back, but you may not be that lucky where you are, today. I haven't seen any more of them around here recently, either.)
If you spin vinyl, one advantage of the 3650 preamp is the flexibility it offers for using different cartridges (MM or MC) at different impedances. I don't know of any other preamp except for certain dedicated "phono preamps" that gives you as many adjustments, and the adjustments do affect the sound quality.
Of course, added features like that mean more components (capacitors, resistors, etc...) internally, so recapping/restoring a unit like that is more work. If you are able to do repairs yourself, by all means get one. Otherwise, find a competent tech and be willing to pay for a good overhaul. This is unavoidable for units this age, if you want the best sound. The good thing is that, once restored, you'll have a great-looking, great-sounding unit that will give great enjoyment for years to come, and --with some TLC every 10-20 years or so, may well last you (and your children!) a lifetime.
These units should also hold their value well, too, for at least as long as there are baby boomers still around who appreciate them, and possibly a bit longer.
Don't shy away from some of the earlier, lesser-powered units, either, like the 240.
I would recommend avoiding the later series which start with letters, not numbers. While not all that bad, really, they just aren't up to the standards of the earlier units. By then, Marantz (under new corporate ownership) was cutting costs, and it shows internally, even if the outward appearance remains somewhat similar.
The exception to later units is the "Esotec" series; these were made in parallel with the cheapened mainstream units, but kept a lot of the quality level and circuit design features of earlier units. Any unit you find that says "Esotec" on it, should be worth getting.
I went the same route, starting with receivers and then moving to separates (although I've kept some of the favorite receivers as a "collection" of sorts, too), and don't regret it. The separates are a step up, especially the TOTL 3650 preamp with the 300DC.
The 500/510 amp series are monsters, ones to dream of, but considerably rarer and even more expensive. If you can find (and afford) one, by all means get it --it's "the ultimate" in Marantz power amplification-- but the 300DC will already give you enough power; it was often used in clubs and such for sustained commercial use, because of its power and robustness. If a reasonably-priced 300DC doesn't present itself, don't hesitate to go for a 240, either. They're easier to find, more affordable, and almost as good.
You said you don't listen to radio very much. You might consider looking for one of the tuners that has a scope, though, just to "complete the stack" and add the coolness factor of the dancing scope! If you really want to complete a Marantz system, look for a 6300 turntable and Imperial 9 speakers, as well, and perhaps a 5025 or 5400 (different, studio-like flat design) or similar cassette deck. If you do want to add a cassette deck, you could also consider a slightly later SD-4000 series, which still matches well visually and offers a faster tape speed (dual speed) and thus slightly better sound. This model was also released under the "Esotec" label, at least in Japan, but I've only seen two of them; one was sold a few years ago on the Yahoo Japan site, and I own the other.
It appears identical to the one without the "Esotec" marking, though; I suspect they stamped the name on it briefly, just to be able to offer a "complete" Esotec series to the market.
