We run into the buy vs. rental question a lot when we are doing Scottish weddings. My first bit of advice has already been given: do the groomsman want to own kilts? Unless they really all want to you are better off renting.

Since you are talking about SWK vs, renting I assume you are not looking at the heavyweight kilts? Otherwise based on your price comparison even buying from SWK would be much more expensive. While I think their acrylic offerings are a good value they are not going to give you a proper formal look if that is what you are going for. A rental kilt will give you a better look.

As far as hose, you can go with inexpensive ones. Your not expecting them to last heavy use so there is no point in getting top quality ones at this point. Pass on the ghillies, they're not necessary unless you can get the use from them. Go for a nice wingtip/brogue, even if you groomsman need to buy them. Every man should have a decent pair anyway.

If you do decide to buy the kilts get the best you can afford. If its SWK, then go for the heavyweight if you can or the Standard if you can't. Then rent the jackets. Sure you could go with the jacket conversions but you come back to whether or not your groomsman want them and also you should have the right look. This is your wedding.