[dominicperry]

The alternative is to keep using your old mac, OS and interface and use an older version of Pro Tools, if you already have a license. Lots of people are sucessfully running studios on that kind of combination. But you still need to check if all the elements will work together.

Buying a whole new computer and interface when you have 3/4 of what you need may not be the best choice - it certainly won't be the cheapest choice.

I second the interface suggestions already made - RME has the most stable and longest-lasting drivers in the industry, with perhaps only Metric Halo being equal, but they are a much smaller (and more expensive) outfit.

Focusrite is very widely used at the budget end. And they do some mid-tier and high-end interfaces too (sound wise). I had some bad experiences with their drivers a long while back (unlike RME, which I have been using for ever with no problem). But recent Focusrite drivers have a better reputation.

Dominic