If you're trying to keep it pretty inexpensive, I would pick up the rifle for around $250 range, at least you can shoot it and appreciate the differences of the upgrades in the meantime. An aftermarket receiver and bolt alone will cost way more than that. As far as the trigger, I put the Volquartsen hammer alone in mine and my daughters and its a night and day difference for only $40... Cut the trigger pull weight in half and very crispy! Of course like mentioned the auto bolt release is a no brainier at like $10 , bolt buffer for a few bucks as well.... Might as well throw in an aftermarket extractor for like $12 while your at it as well...
Of course if you don't really have a budget, plan on changing everything including the bolt and receiver and dont mind not being able to shoot in the meantime then definitely go all aftermarket... Myself however always wanted/admired the 10/22 legacy and like the fact that at least the receiver is Ruger lol... Stock Bolt and receiver works perfectly reliably for me. Maybe some differences if you're in competition and every 1/8" makes a difference but for plinking and fun Id say they're more than fine. Barrel and trigger hammer made the biggest difference in accuracy for me... Cut about an inch in grouping for me... I may possibly change the bolt and trigger assembly down the line just for the sake of upgrading but I wouldn't have had it any other way, the progression and the experience of starting with the 10/22 itself was priceless for me and unless I change the receiver which I don't plan on, my builds wouldn't really have costed any more than if I started from scratch anyway and I wouldn't have been able to shoot it and see/feel the progression for a few months!