Do a little searching, i.e. the search function is your friend. Special Weapons stopped making complete guns years ago, but until very recently made receivers and/or barreled receivers marked with the SW name. Because of this, there are many questions that need an answer. I could be wrong but, as I recall SW switched over to Coharie Arms about 10 years ago. But the owner, Todd Bailey, continued using SW name for rolled and welded receivers. So is the gun an original SW5? What does it say exactly on the receiver? What is the model #? Are their any markings on the barrel? I have two full size 9mm guns made by putting HK MP5A2 parts sets into SW receivers by a "name" smith that are worth around $2,500. On the other end of that spectrum, some one could have gathered some "factory seconds" (Todd was know to cut up the guns that were built too far out of spec and sell the parts on Gun Broker) and welded the gun together himself with a SW bare receiver.
Todd's early guns didn't have flutes in the chamber. Does the gun you are considering have a fluted chamber? If so how many flutes are there in the chamber? The flutes are only required for function in rifle calibers. There seemed to be enough demand in the market to justify fluting the chambers that eventually Todd started fluting the chambers in the barrels he made. What does the bolt look like? Does the bolt have any markings? There will be a lot of things you'll need to know to buy a custom build. Just because it says "Special Weapons" on the receiver, doesn't mean that Todd built the complete firearm. Todd has sold rolled and welded receivers, barreled receivers, with no finish, barreled receivers with finish, barreled receivers with finish and bolt groups, and complete firearms with the SW name on them.
As far as the Vector .40 caliber noted above, Todd made a bunch of .40 S&W kits for Vector. I never got an exact description of exactly what happened. Some say that the kits were way out of spec so Vector rejected them. Others have said that one of the principal owners of Vector got into a legal scrape concerning some 7.62X39 ammo supplied to the Iraqies and used the money that Vector should have used to purchase the kits for his legal defense. At one time Vector was down to one guy just doing warranty work. Before then Vector was know as a quality roller locked manufacturer. Since that time Vector's QC has been less than stellar. Whatever the reason, the .40 S&W full size kits were sold at $400 each. There was even some .40 S&W SD kits sold with a .Rim Country Machine manufactured (but Vector marked) .40 caliber SD can with all anodized aluminum internals and tube for $800 including a Vector marked receiver. The full size .40 was missing the firing pin and the SD was missing the rubberized SD hand guard. It is my understanding that if all the parts including the receiver is sold, that would be a complete firearm and subject to 11% FET.
If this is a local deal, meet up with the seller and see the gun run. If it does, whomever built it, doesn't matter so much, at least to me. A running gun is what you want. Seeing it run would be the most important thing. I hate to say it but some sellers have lied as to whether their roller locked gun actually runs. Roller locked guns are big money. I've seen post after post of "this thing is making me crazy, and I want to give up and sell it". So if this is an out of state deal, and you don't know what to look for, this could be a nightmare, if you expect a working gun and you don't receive one. This could be a great way for you to learn the roller locked system. But if that is what you are willing to do, I would consider the deal as you are buying a collection of parts that might or might not run. You also should consider that if additional parts will be required to make it run, you have funds in reserve for such parts, and possibly the services of a HK smith to make the gun run. There are smiths that offer this service. Again, I've seen multiple posts where the newbie "borrowed from Peter to pay Paul" to purchase the roller locked that was supposed to run, and doesn't. Now he doesn't have the knowledge and/or funds to get the gun to run and is crying"What should I do?". What I really want to post is you should have thought of this and been more knowledgeable before buying. But that would be mean to rub salt into that wound.
This post is pretty long. I've only scratched the surface. There is no way I can give a real answer to your original question. My first roller locked was a Vollmer converted 94 that was "married" to a Fleming sear. I didn't need to know much about roller locked guns when I bought it. I just put ammo in the HK mags that came with it and it ran. I didn't really start to learn the roller locked system until I bought my first clone from Todd. I've bought several complete Coharie Arms guns and some SW barreled receivers. It took a while to learn the guns, the things about the guns from Todd that were different from factory specs, and what I needed to do to get them to run. Maybe the gun you want to buy runs great. If you get to see that, you are ahead of the game. But if you can't, assume the worst and hope for the best. Good luck with your MP5 clone purchase. YMMV.
Scott