We're not supposed to discuss pricing in this forum, so I'll just stick to the vitals and my impression of each herewith, without assessing the fairness of the prices quoted in the OP
2000 and 5000A are fairly closely related. Both are late 60's tech, capacitor coupled amplifiers of the same basic underlying philosophy. 5000A is 55WPC output max and when in proper order sounds very good, has a "punchier" sound than later Sansui receivers by virtue of being cap coupled. The 2000 is less output power (right around 25WPC by my recollection) and is of a slightly older aesthetic design that was updated to look more like the 5000A with the 2000A and later still the 2000X. The latter two each got slight power boosts over their predecessors.
551 is early 70's, good build quality. It is middle of the range for its series (221-881). This line is prerty well regarded as sort of a sleeper series, since they predate the 5050-9090 and the horsepower wars, so aren't as quite powerful or as feature laden as the successors that came out a couple of years later. You don't specify the severity of the cosmetic problems, but that is usually a deal breaker for me as I prefer gear that displays as well as it sounds. If the issues are minor, maybe not a problem for you. Kind of a subjective thing.
The 5050 is the smallest model in the series 5050-9090, mid 70's. A decent unit, sounds good but the entry model within its lineup versus the 5000A which was the top of its line, albeit a few years earlier. Major features and power about on par with the 5000A and is certainly more "modern" looking in a mid 1970s sense for layout and styling versus the decidedly late 60's look of the 2000 or 5000A.
Of the bunch, if the 5000A gets to be fully working and isn't outrageously priced once done, that would be my choice. I own one and really like it, even with a lot of newer more advanced Sansui gear at my disposal.
Of these the 551 is the only model I don't own at least one of within the same series. I have 2000X, 5000, 5000A, 5000X, 690 (black front panel version of 6060, one up from the 5050), 8080DB and 9090DB. So, it's the only one I can't personally speak to on sound quality. Other members here have posted favorably about the xx1 series numerous times, including one recent thread that gave very high praise to a 771. I've no reason to doubt the veracity of those claims.