Definitely stick to designated NAS class drives, cheapie HDD can get destroyed with the work stresses .
Read the manual for the NAS and look for upgrading/replacing drives as some make it a bit tricky or restrictive. Personally I recommend getting the largest drive you can rather than cheapest (smaller sizes). I'm in the process of replacing all mine with 8TB drives as my growth needs grew faster than expected. Fillling the four slots should be doable over time as you can afford drives. Also if possible if it offers some form of hybrid raid formatting try to choose option that gives you one drive redundancy.
Whatever you do, choosing drives for the NAS these are what to look for in order:
1) NAS rated/class drive eg Western Digital Red
2) Capacity as largest you can afford and the NAS supports, 8TB may be a good price point and been around long enough for NAS updates to allow support.
Longer term aim to fill the slots with identical drives, prices generally drop over time so you should be able to expand as you can afford.
3) that's it really. Take a look at the features the NAS offers like some form of App Market that lets the NAS also take on other roles like a streaming media server, web server cloud storage etc and if appealing make sure to choose the larger drive size over the smaller.
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