Hmm, for laptops, I almost always recommend a slightly used MacBook Pro i7 quad core(a few years around 2011 offered both firewire and thunderbolt IIRC). PC laptops can be a crapshoot with compatibility(but if you can get an exact model from someone having great success, then that's a pretty safe route). Desktops open things up as there are some killer PC builders(pro-tools-pc.com is one of the best). Or a slightly used Mac Pro 6 core. Just a note re the Mac/PC debate. Both platforms are capable of delivering great performance(and both are also capable of making you rip out all your hair
). PC can save you lots of cash. Mac MIGHT save you some hassles(but might not).
Another detail that you need to be aware of is compatibility with your old sessions and a new system. On a Mac, PT10 is the newest version that reads SD II files(if your old sessions are in that format, I'm not sure if PT11/12 will convert or not). SD II files also will not be read on any PC. Lastly, I'm not sure when it changed, but earlier version's sessions were not "cross-platform" without being created or saved with Mac/PC compatibility checked. Somewhere around PT7.4 or 8, that detail became a default(I often move sessions between platforms without issue).