do you have one or thinking about it
If not then you would be best thinking what you want to build it for and how much your happy to spend
ie if you buy a std Donor gun and want a high end build you will pretty much chuck every thing away and replace with relatively expensive parts This can make the receiver (the serial Numbered part) quite expensive so you might be better off buying an after market custom Receiver for example from Rimfire Magic who make their own and specing up the parts you want from them etc (also see Rude Fat dog for KIDD parts etc)
if you just want a tinker and slash a few ££ on it for a bit of a project an Old cheap second hand gun can be great to play with and still end up with a good shooter (See you tube for lots of 10/22 modding )
losts can be done to improve what you have ie you can fettle the trigger and smooth out the Bolt etc with hand tools if it all goes wrong you just buy the Upgrade parts
Barrel change can make a big difference but can be expensive as long as you hand over the old one to he RFD as he hands you the new one no paperwork is needed FAC wise .. Taking the barrel band of a std Gun helps a lot you could get it recrowned etc you may pick up a heavy barrel versin thats fine a lot fit a Green mountain Barrel as a mid price range or you can sell a Kidney for a Volquartson or KIDD barrel
there is also hundreds of after market stock about in any style you care for and lots of second hand ones
I picked up this old (1994) lady at my local RFD for £140 as it had been raced Hard Im guessing Mini rifle it had already had a lot of the Slicking and fettling done and the std barrel chopped to 12" just needed a Good clean
changed the stock and a new barrel a Volquartson 12.5" carbon fibre wrap added a Moderator runs great on Subsonics as a lightweight hunter
seems to have settled on CCI Subsonic Hollow points
next i think a strip down and repainting the receiver and trigger unit its a bit battered