jogaklaa

& 5. I've never bought them off Etsy before, but it should be perfectly safe, especially with the reviews the store you're looking at has. The main concern is how long the link will be available for. If you can I would get them off cults, they host the files and 3D files is their entire business. I've used them before and have had good luck with the files. Doing a quick Google I'm not seeing this user on cults though so you're probably stuck with Etsy, but that should be fine.
The easiest file type for printing is .STL and pretty much any print file will already be in that format. You should be good considering this was made to be 3D printed on an Ender 3.
Only the arm piece I'd be worried about not fitting on a print bed, it seems a bit long. Bit the listing does say it fits an ender and specifies the volume of that printer which is a pretty standard volume. You'll want/need multiple print batches to get everything done but you'd want to do that anyways.
I think most professional places charge based on time and material used. I honestly couldn't say how much that would be. I think a decent amount of the online print shops have a quote estimate to give you an idea. You also might live in an area with a library/community maker space with printer you could rent, they tend to only charge for material (kilo is $15~20) but it might be a little harder for a good quality print of the bigger pieces. They'd also have people there you could talk to about the whole process which is always nice.

One other thing you haven't mentioned is post processing a 3D print. It'll be a good amount of priming and sanding for that so be ready for hours of that.