It does depend on whether you simply need a 'knockabout' for a short period of time, or are potentially investing for future use (whether with GIS or not). It also depends on what sort of GIS work you will be doing: viewing vs geoprocessing, vector vs raster, 2D vs 3D etc.
For lighter GIS work, a previous generation i7 should be sufficient - or one of the later i5s. Don't go back too far in the generations i.e., stick to the last 2 or 3 for i7; or last 1 or 2 for i5. Use CPU benchmarking websites to help guide you. I really doubt you need the latest generation machines for this specific use-case - certainly not the latest i7s. You may also get away with integrated GPU as opposed to dedicated - bearing in mind Pro is capable of 3D work, and not all users will be doing this. But the gamble with an under-spec (which is what this would be) is that it will be on the slippery slope from a 'bit more laggy' to 'unusable'. If you take the gamble: keep the receipt.
SSD - the lack of an SSD won't break Pro, it'll just make it slower. You can go HDD if that's what it's sold with, or a smaller SSD if you know you won't max it out with data. You may find a better drive elsewhere that's cheaper than the difference in cost from the retailer - if you're happy to swap-out.
RAM - wouldn't go any lower than 8GB. If you do look at older machines, bear in mind that motherboards can only support up to a certain amount of RAM. If you know you have a machine that can support 16/32GB RAM, then starting on 8GB allows room to expand if you need to*. Buying an older machine that can only ever support 8GB doesn't give you any wiggle-room. This may or may not be important, depending on your long term intentions with the machine.
* if this is something you would consider, then start thinking now about how many RAM slots you're getting, and how distributed the RAM sticks are. You may find 8GB means two slots with two sticks of 4GB, meaning you have to buy 2 x 8GB to double your RAM. If it has two slots and one 8GB stick, then the same upgrade is half the cost. If it has one slot and one 8GB stick then brace yourself, as the single sticks get very pricey as you go up in size.
But before parting with any serious cash... I would recommend dual-booting or virtualising Windows on the Mac, at least to start with - to see how acceptable it is. If not, you could then use cloud-compute services such as AWS EC2 to trial out a few different instances to work out what specification gives you the best performance for your needs - like a 'try before you buy' (but at an hourly cost however). I wouldn't advise using cloud-compute for the semester itself however unless you have a really clear idea of hours, and a good memory to stop the instance immediately when it's not needed. Otherwise, if could cost you more and you get nothing out of it!
