The main key for getting a separate preamp are 1) accommodate MM or MC cartridges if you amplifier do not have MC abilities 2) may provide better sound quality and flavor. I don't have a separate phono preamp as my amplifier can accommodate both MC and MM. But, they're nice to have and can add some flavor to your music, works just like a standard amplifier. It's really up to you if you need one or want one. I say if you don't have a MC then don't bother. However, if you do have a MC and your amp does not have MC abilities then yes, by all means. But, I never heard anything from a phono preamp before so I can't comment.
Flow Chart:
Turntable -> phono preamp -> amplifier -> speakers
Since you already have a phono preamp just focus on getting an amplifier. I don't know what a RIIA box is so...yeah, I'm a bit confused. If it is another phono preamp then it's up to you to find one which one suits your taste.
250 Euro for a vintage belt driven 505? That's pretty expensive imo. I don't know the price that vintage tables go in Finland, but that's too expensive for a 505. I've seen a 505 here in Canada for an average price of $75CAD. The 505 were decent tables but nothing special about them. If you can, find a vintage amplifier or receiver with both MM and MC abilities if you don't want to spend extra for a separate phono preamp just yet. But, pricing is up to your area. But, 500-750 Euro is a lot of money, you could probably buy a top of the line vintage piece for that kind of cash.
Hope that helps.