Don't buy a raven barrel. They're so called "B-grade" barrels, not very good. I'd buy from a respectable "budget" company or I'd just go ahead and buy a proper PDI barrel right away. Raven barrels are coated carbon steel barrels, I'd go for a stainless steel barrel if I were you...
The PDI trigger is fine, but you can get cheaper triggers that'll do about the same thing, so if budget is an issue, get a cheaper trigger, don't get a cheap barrel.
Do NOT leave the hop up as it is. Everything but the hop up, lol. The hopup is the FIRST thing you should upgrade if you're looking for performance. The stock TM chamber is alright but there is better out there. I'd recommend an action army chamber.
Much like #2, the PDI cylinder is good, but again, so is an action army cylinder for a lower price.
Long story short: PDI is good stuff, but there are other options out there that are just as good for a lower price
You can use the stock outer barrel and receiver (most people do that). You can either buy or make barrel spacers, it doesn't make a huge difference. The VSR pro rifles are a bit of a hassle there because the barrel is tapered, but it's very doable. Applies to both bought and made spacers, by the way. Tapered barrels are simply annoying (unless they're straight on the inside).
As for springs, get an M150.
Since I want a durable replica without having to worry about parts not fitting I have decided to go full PDI
I have a combination of airsoftpro and action army parts in my VSR and I have had zero fitting issues. If you go with PDI you won't have issues either but it'll just be more expensive and it's not really worth it in my opinion. I'd advise against it and use action army parts instead.
I'd recommend this:
-AA trigger (about same price & quality as PDI)
-AA cylinder (cheaper & arguably better than PDI)
-AA hop chamber (the best out there)
-PDI barrel (PDI makes good barrels)
-PDI W-hold bucking (decent bucking)
-AA trigger, cylinder & hop chamber
-Edgi barrel (non-bridged barrel) (Edgi barrels are the best barrels you'll find)
-Maple leaf bucking & concave nub (the closest thing to an R-hop without actually doing an R-hop)
The maple leaf bucking will give you better performance (and so will an Edgi barrel) but it's probably going to be more expensive since you'll have to import them.