Hi, Dee. Welcome to the forum!
...but even then it gave me no option but to choose seat. only then on the 2rd step it let me choose sleeper.
As a guess, they simply struggled with correct translations to eng, thus you first choose seat (as place on a particular train) and only then seat (as seat in seating carriage) or sleeper as a type of this place.
now i have chosen seat 51 as that is what was available. can somone please help me, will this definatley be a sleeper or not?
Yep, definitely. It's a lower bed in 3 bed compartment. First digit signifies compartment number within the carriage (thus you have compartment #5), while second number - particular bed in this compartment (in your case 1 means lower berth)
is it better to book online like this? or is it better to do it at the station?
Well, Bulgarian Railways (BDZ) started online ticket sale very recently, it's still a pilot project with limited number of trains for which you can purchase your ticket online. Also it seems that there is only certain quota of seats/beds for sale online (hence they warn you what "Contingent of seats" is available), while the rest you can buy more usual way - at the ticket counter. As far as we know, during off-peak season it's not a problem to purchase ticket even on the day of the journey (thus you can wait and buy your ticket already in Bulgaria via ticket counter), but for the summer season online in advance purchase might come very handy
and if i do book online how am i able to collect my ticket? will it be from the station?
It's a e-ticket, thus you print it yourself. BDZ even on the website states that after you pay, "print the electronic tickets and have them with you when traveling", so you won't need to worry about collecting/printing them elsewhere