Hello and welcome to the forum.
Eheims are considered by a large number of our mebers to be the best external filter available. I have 2 Fluvals on my 75 gallon and have been very satisfied with those
but some consider them not to be as good as the Eheims.
People like me for example, who have had electric shocks off Fluval exturnal canisters, suffered high noise levels and less than great performance from the outset. I also have a mate whom has had one leak. Personally, having ran Eheim Ecco's, Fluvals, Tetratecs and Renas, The first two are the only ones I would leave on the shelf...

Tetratec's and Rena's are goos filters IMO, though so-fay the seals of the Tetratec's have been better than Rena's...
The Ecco's are made under the Eheim brand name by JDA, one of Eheim's sister companies, and they are not a scratch upto the other Eheim canisters that are made by Eheim apparently... Some people have 30 year old Classics that are still going strong, so they are deffinately reliable. Relative to modern filters though, Classics (and Fluvals IMO) are harder to prime than most competitors...
In a 50g, I'd be considering;
Tetratec EX1200
Eheim Pro2 2028
Rena XP3
In that order...
You want a Classic model Eheim, but they aren't realy something that I would likely buy... If they still interest you, a 2217 would be best for your tank
All the best
Rabbut