If you haven't deployed yet, i think it is a good thing to get a general idea of what your looking for, how you want to do it (Air or liquid, etc) but you'll be gone for 6+ months depending on your service / unit. in 6+ months things in the tech world, especially in the enthusiast sector of computers, things change fairly rapidly, the best MB on market now, could be 5th best or less, in that time, and may or may not meet your expectations.
Things I would start thinking of before parts are, what do you want to do? game, benchmarking, video editing, etc. you'll have a nice chunk of cash saved up after you get back, dont let it burn a hole in your pocket before you even get it.
you'll have plenty of time and resources to decide on the parts, but the design process is the fun part imo. for example if you choose air, what case will you want to use, and how where will the airflow go? imo liquid is more detail orientated, so if you decide that route, what type of fittings? single / dual loop or more? internal / external rads, most importantly how do you want to setup your loop?
these are the questions i would be asking myself before parts, once you decide this finding the right parts will be much easier.
thank you for your service, and good luck!