Hi Ricky,
Might be worth checking over the setting up on your existing machine before splashing some cash, if you haven't already.
Probably worth fitting a new blade from a good supplier, and if cutting straight is the priority, make it as wide a blade as the machine will take. Also, unless good finish straight off the machine is important, use a low tpi say 4,5 or 6.
Then make sure the blade is adequately tensioned.
Then make sure the guides are adjusted correctly.
Any of the above could give problems if not sorted properly, even on a new machine.
When you say it doesn't cut straight do you mean you get a bowed cut (top to bottom) in deeper timber, or that it won't follow a straight line along a piece of timber ?
Bandsaws aren't the best for following straight lines anyway, even if properly set up due to the inherent design of them, and if this is the issue you may want to try a point fence rather than a rip fence. Think somebody described this on here recently so could be worth trying a search.
Cheers, Paul.