For modifications I would suggest not using Ebay. You keep saying eBay this and that and the thing is you can get really screwed on there.
As for a Boost gauge google it and I bet there are a bunch.
www.amzperformance.com has a few listed and so do many other sites. If you can't find one for less then 200 something is wrong.
You shouldn't need anything extra everything should be included in the kit.