SHS steel sear set- about 15 quid from ehobby
Dangerwerx type B lever- 20 quid from ASPUK
Your piston will get eaten by the sears. So I'd buff up the budget to 60 quid so you can afford an Action army piston from ehobby or somesuch for about 20 quid and still have a tenner left over for supplies for DIY mods.
DIY mods-
Seal hop up buckings with dental floss
Seal cylinder with PTFE
Grease the gun properly with some nice silicone grease. The earwax stuff JG uses is awful.
Thoroughly wash the hop up buckings in fairy liquid
do the google dance for more class DIY mods. Plenty can be done on the cheap.