It really depends on what you want. I have been playing trombone for about 8 years, and I started on a really basic student level horn. I then moved to an intermediate horn with an f-attachment, and finally to a Bach 42BO symphonic trombone. Then I started playing bass trombone for Jazz, and eventually for Symphonic as well.
In your situation, I would suggest the valve trombone, as it is basically the same as a euphonium. Same valve combination's (unless you are playing a 4 valve).
If you want to get a real t-bone, go for one that is a slightly smaller bore, without an f-attachment. Ska demands some pretty high notes, and you will blend with the other instruments better.
How much? A good concert trombone is about $2,000, but I would say something in the ~$900-1200 range would be good. Looking for used ones helps some too.