I'm thinking it might be better to get the whole rifle first, just because not all the parts may be readily found, and if they are, it would be the more expensive route to go. Like buying all the parts to build a new car, it's going to cost alot more. Some of the parts don't have an upgraded version, like trigger guard, buttplate, tang...so you would be spending alot of time and effort trying to locate just a few parts. If you bought an old beater from the classified adds here, or ebay, or some site like that, you can replace the "high performance" parts, a little at a time if you choose, but you would already have all the other parts. You could change out a couple of key parts on your rifle, say the barrel and triggers, and still shoot it without having to post, "Hey guys, where can I find a cheap T/C trigger guard and tang"? You can go for the stock later, maybe even the lock if you choose. BTW, if it's a percussion lock, I can't see any reason to ever change it out. Good locks with lifetime warrantee.
Once in a great while you can find an entire set of used T/C hardware for sale cheap. I built this little rifle from a used set of T/C hardware I found on this site. I got it for a song and a dance. The set included the lock, trigger, trigger guard, key escutcheons, key, buttplate, tang, and all the screws and bolts. However, it took about a year of searching all the sites every day, so I wouldn't count on it if you're in a hurry. :surrender:
Just my opinion. Good luck, Bill