To take advantage of tethering with a P+, back, you need firewire, so that puts you into a Mac hardware platform. I guess there are some windows laptops out there on the used market that had a firewire port, but for your needs I would consider staying with Mac.
Options, are to pick up a older generation Macbook pro, with the faster firewire port, you can get cables that have the slower style on one end and the faster port on the other. You will need to get an older version of C1, than 9, but these you can still download from the Phase One site. Version 8 should still work fine a Macbook pro that is running a version of the OS, prior to Yosemite. I would make sure to have 8GB of ram installed at least in the Macbook. This will allow you to tether and get the files across to your laptop with no problems.
Any of the older 2.4Ghz i7 (models in the late 2011) 15" should work as they all have the Firewire port. You can also look for an older macbook Air but many of the early models only had 4GB of total ram available. The 15" macbook pro models will be the non retina display.
You can also install bootcamp on this machine, and then run win7 or possibly win8 or 10, but odds are the boot camp level at this version of the Mac OS will not have driver support for the later win OS. You can also upgrade the Mac OS to El-Capitan on the 2.4GZ machine and then install C1 9.0x. Lots of options.
I have a 2.4Gz i7 which is running El Cap, but I only have C1 8 on it. C1 8 should do fine with the P30+, but you may want to try C1 9 as there are some improvements in the processing engine that appear to help the older P+ backs and early IQ backs.
Bootcamp, is easy if you are familiar with installing windows, if not, I would consider just running in the Mac world for tethering. Mac is not a bad thing at all, I run win7 on most of my machines, but have 2 Macs, which I use often for tethering needs and quick processing.
The Microsoft Surface 3 or 4 would be a great product, but as it's only USB3, you can't go there with the P+ backs.
Paul C