Get a Prometheus barrel, you'll gain much more noticeable results combining with your suggestions. Although, you'll probably get better performance replacing all the stock internals with mancraft parts.
This has been answered hundreds of times, you can use them, there is nothing wrong / illegal with you owning RIF's. If you want to purchase RIF's however, you need to be on a database which retailers can check if you're eligible (UKARA)
£50 for a 3D printed KAC silencer, you can get decent ACM ones for under £30..
http://forums.zeroin.co.uk/showthread.php?266739-Barrel-shroud-suppressor-your-14-5-barrel-to-10-CQB-in-seconds
Jeez, here I go again.
M140 spring
Steel cylinder head
Steel cylinder
Steel barrel + hop up housing
6-24x50 Scope (current scope doesn't zoom or adjust, which is a completely pointless.)
Also nabbed from ebay:
Heroshark speartooth goggles for £17.50
1000 BBKing .45g BB's for £13.99
No point. Expensive spring shotguns will perform the same as a sub £40 single-shot shotgun. My DE M47 could reach out to 40m accurately against a steel barrel, it did feel a bit flimsy though.
Get yourself a Marui M870 or a tri-shot.