Which focal length to get is a matter of personal taste.
If you do want to get a 500 mm, and funds permit, try to get the Tele-Apotessar (the CF lens), and not the Tele-Tessar (the C lens). The Tele-Apotessar is decidedly better than the older Tele-Tessar. And it has internal focusing, keeping the shift in balance during focusing very small.
The Tele-Apotessar comes in two versions, differing only in minimum focussing distance (8 m vs 5 m).
The other two telephoto focal lengths (250 mm and 350 mm) both offer a choice between a 'regular' lens and a Superachromat (the non plus ultra in colour correction).
The Superachromat versions are, of course, better, but also much more expensive. The 'regular' ones are quite good also.
So a choice depending on the content of your bank account. But not one that leaves you lumbered with a bad lens should your bank account not allow a Superachromat.
All (!) lenses should be used on a tripod if you want to know what they are capable of.
But while you could get away using the 250 mm handheld (a waste, though, of the money you spent, if it was spent buying a Superachromat), you absolutely must use a tripod with the other two.