[splindoffi]

drivetrain, casette, rear deraiuller, front fork. Theyre all about over 300£ ish. ( proper mtb stuff  ) whats a good weight for wir?

There's no specific weight for a WIR, but the two items you're going to buy must match as close as possible an exact weight.
Also, picking an item from the same kind of item you want to SE improves your chances of succeeding.

by two items what do you mean?

With the WIR method, you have to order two similar items from the same company (your SE item and a cheaper one), on two different accounts (not linked to each other) and being shipped in two different addresses. You'll claim you received the cheaper item - The company will ask to return the cheap item and will refund you the cost of the SE item.

Items should match in weight and also in category (if possible).
So... let's assume you're going to buy a rear derailleur. I bet stores got both cheap and expensive ones.