I will say this... I have never had a single issue purchasing from sellers I found on the TDH. I know some people have had issues with some sellers on here. I would highly recommend limiting exposure by doing commissions piecemeal. There are a lot of pieces to this costume. Helmet, soft parts, boots, jet pack, armor, gauntlets, EE-3, Pulce, holster, etc. can all be broken up. Yeah, you might pay a little more in shipping. But it is worth it to know that the most you will be out is $300 - $500. I did a full JP once for $750 and a brass cast of a Webley for a little over $1k, but that's the most I have spent at one time.
I can't imagine parting with $5k and hoping for the best. That is too much to put out there. And for vendors living hand to mouth, that just invites the opportunity to live off of the next full commission. You are creating an opportunity for someone to dig themselves in a hole they don't want to find themselves in. In fact, I believe that a lot of the people who have turned into crooks on here started out with good intentions and found themselves broke and without motivation.
Another purely selfish reason to do these things piecemeal is the joy of unboxing a new toy. You can unbox one big toy. Or you can unbox a ton of little toys throughout the year. I personally enjoy unboxing all of the individual parts. I end up having a lot more appreciation of all of the little parts when I do it that way. But that's just me.