| +1 to waiting if you're not in a band yet. The backline 600 is a decent sounding powerful enough budget amp to gig with, the 250 is pretty weak. I don't think the backline cabs are all that great either.
I'd just put the money aside and earmark it for a gig rig in the future.
The key to getting good deals on good used gear is to obviously know what you're looking at and what your needs are, having the money put aside for when the right deal does come along, and employing patience. Watch stuff come and go until you see what you want. It wouldn't hurt to buy a gig size rig even if you don't need it right now if the deal is right. You will need it in the future but I don't think this one is it.
Keep practicing. A 90 watt combo should be big enough to jam with some friends in a garage or basement so get together with some folks around your same skill level and have fun.
Of course you need to practice by yourself to get better but it's not like playing with a group. Keeping a band together is another form of practice that is invaluble to a good bass player. So is being able to sing and play at the same time. Getting together and jamming will be good for everyone involved on all the different instruments. It's also a lot of fun. You'll become a better player and it won't "seem like" the practice of sitting down by yourself and running through scales, etc. | 
