Hello and welcome, there is A LOT of info on all the forums and it can be quite daunting, the Getting Started Area is the best place to start. Finding out what version of Stormtrooper your Dad would like to be as there are a few different ones, here are a list of the CRL's (costume reference library) which are the base requirements for legion approval https://databank.501st.com/databank/Costuming:FISD_CRL
The helmet you have shown is a Black Series which is not approvable for the original trilogy costumes, and would need a lot of work to get it up to approval for Rogue One version, here is a conversion build
Always best to purchase from the vetted sellers list, you can be assured of quality and not being ripped off on ebay or esty, maker contact info can be found here
Best practice is have a look at some of the build threads, most have the makers initials in the headings, there you can see what you are buying, Another great idea is see local members in person, many have armor workshops and it's a great way to get hands on products and info.
You can add as much as you want or as little into the interior of the helmet, as long as it's not seen on the outside.
Here is an all in one resource thread for anything else you may need.
Why are they called TK's, well nobody really knows the exact answer but:
Research is the key to this hobby and it's not a cheap hobby.
Hope that has help answer some of your questions, good luck with the research and hope to see a build thread form you soon.