My .02 was going to be buy the new interface now and update your current computer, see if everything still works great for you. If it does, then you're golden, if not then its time to upgrade. You need the new interface regardless of whether you get a new computer or not, so it makes sense to do that first. At that point you would just have to decide if its worth spending any money on the old computer to try and scrape by for a while longer, or just go ahead and invest in a new machine. My Mac at the rehearsal space isn't quite as old as yours, but its a 2012 Mac mini and its still doing a bang-up job no matter what I throw at it.
My big thing on Mac vs PC, I've had multiple Macs last me 5+ years and in every case I've only stopped using because I either broke them (beer + keyboard doesn't mix) or just upgraded because I had the cash and wanted newer nicer things. I don't really know of anyone with a PC machine that gets a similar life-span--anecdotal, and I know someone will chime in with a counterpoint, but that's my experience. That does NOT count the desktop tower crowd that can remove/repair/replace each and every component in the system and might go through multiple hardware upgrade cycles over the life of their machine. I'm talking about laptops or all-in-one desktops that are basically sealed units like an apple system is these days. Sure, I can't upgrade my MBP beyond what it was when I purchased it, but I also know that without having to put another dime into it, this computer will easily last me a decade or more. Seems expensive at first, but it seems like when people buy the alternatives they're spending 2-300$ on a PC notebook every 1-2 years. If my Mac cost 2500$ and lasts me 10 years, that's an equal per-year cost, YMMV.