i do have a 3d printed shoretrooper and I can send you links to acquire the STL's. some are free files but require a little work to make them more accurate, some people have updated them since but take a little to find. and there is an expensive one but the modeler worked with a large team to make it super accurate like you walked off the set, they even modeled the interior of the armor that was never seen.
here is the link, it does includes Pauls like for accurate armor and sean fields files which I used for mine that are free but have to look at CRL (costume reference library) and compare to STL before you print just to make sure that detail is present. shoretrooper.co.uk/
also includes the facebook page where you can find remixes of the models or other useful information.
3d printed armor is allowed as long as it doesn't have layer lines present on seen parts of the armor (inside is fine) although durability is always a question it can hold up really well. but obviously not as flexible and ridged as ABS vacuumed formed kits or molded casts. but there are ways to reinforce it with fiberglass etc..
softparts there are a few vendors that are trusted that make custom clothing. mines from imperial surplus, more expensive but include everything needed, but not most accurate when i got mine) darkside closet does really nice work (https://www.facebook.com/groups/1742353536003671/user/100064456161791/) and some more if you want them.
if you need help finding any items i am able to provide more details if that is what you want to build. or for another character i can help you find kits/stl's/softparts.
I am still working on the helmet. opted on the Niko Henderson helmet, apparently its the best one out there.
i plan is too hopefully go for the Mr Paul's kit (although yikes it is pricey for something that doesn't have everything required). but doing some research i would need to do major mods on it and it might not fit the way id want it. ill have to check it out some more.
heres where i am with the helmet. got everything printed in PLA except for the front detailed piece, i resin printed that to keep the details. ive started to sand and once i get it where i want it, ill put a few coats of resin on it and sand it down.
Paul's files are the most accurate files on the market, he has done so much research and has help from actual designers of shoretrooper. no mods should be needed for accuracy. he even has the internal details done on inside of the armor for strapping and everything. his armor looks no different then screen used costumes and its insane, his paint work is fantastic. But they are super expensive as it goes to charity.
fitment is always an issue they are designed for one size and things will have to be scaled to fit.
Niko helmet is the best, next to Pauls newer helmet but he hasn't released it yet.
Free files are Sean fields that is what i used for mine, but those will require some mods to include details that he missed that were added to our CRL later as more reference were found.
some people have added modded files and should see around the facebook build ground forums to fix inaccuracies but you have to look around and compare his files with CRL to make sure everything is there before you spend time to print.
really depends, i like to sand the complicated areas that are going to be hard to reach when assembled, then rest are light sanding all around before i assemble. this also helps glue adhere better as it likes sanded surface to grip into vs unsanded plastic. majority of sanding in when its assembled.
that way you can tackle the seams as to make then flush.
depending on the filler you use some times its easier to sand filler resin/bondo/filler primer etc is easier to sand then plastic. so after initial sanding of defects and small inconsistencies then i add my filler/sand/filler/sand until layers are gone if there are small stuff left I will use spot putty as it dies quick and easy to sand but too thick and it crack apart.