TXC 450 Top End Rebuild

Hey guys, new to the club of owning a Husky. I just purchased a 2008 TXC450 and I have to say it's an amazing bike. I went through the process and got it street legal. Took it in to my usual mechanic and he said it is probably due for a top end rebuild. Previous owner raced it. In order to keep it running it has to be fairly hot, and a little white smoke when you first start her up. Does anyone have any tips on knowing if it is time for a top end rebuild?
He quoted me for $800, parts and labor. He usually quotes a little high.
Does anyone know where I could source the parts myself? And how intricate the process of a top end is? I have very limited mechanical skills. But I would like to be able to do this myself one day at least. Would this be a big project to tackle?
Thank you

Also, if anyone has the repair manual it would be awesome to have!

This is a good time to pay for a good wrench to do a leak down test. You might need several different things rebuilt, valves, top end, etc. Id recommend going down to the rod bearings if its coming apart for a top end. Deeper job but your starting fresh. Id hate to see you ten hours after a new piston hearing a rod knock.

Do you know where I could source the parts? I haven't found any sites that offer them. I did find OEM parts but I believe it was around $200 just for the gaskets. Thank you for the help

related FYI not your answer but related, mine (2008 TXC450) had @ 300 hrs on it, AMA Western HS, Tecate Enduro/HSs, National H&H and countless(counted) hard core trail ride time.
My maintenance was very good along with all my bullet proofing mods and special parts. 7602 carb vent filter or other brand is great to have on these FCR carb equipped bikes.
so at rebuild in 2010, both my exhaust valves were beat up on the seat faces we replaced these, we spec checked the intakes and reused them, all wear was minimal...cams great, along with all plain bearings. We split her, rebuilt the crank all fresh and blue printed and installed all new bearings wherever they lived, we also installed the SM counterbalancer complete assembly(the best mod ever for these Italian 450/510 TC/TXC engines), clutch and basket replaced, new piston set, gearbox was at almost zero wear with the exception of the shift barrel, which was worn, we replaced it , its alloy and wears. The evidence of very low wear I attribute to good moto oil changed on a regular basis (Maxima happens to be my choice) and SS oil filter.
Bike is owned by friend and from now back to rebuild in 2010 has low use hours maybe 100.

I appreciate all the info guys. I hope to have this bike for as long as possible. It's a complete enjoyment to ride. Where can I find the SM counterbalanced assembly? Do you know what it costs?
Robert, thanks. I'll definitely make sure the parts you mentioned that were worn are double checked.
The bike was raced for around 3-4 years before it ended up in the hands of the previous owner. Who maybe put 20 hours on it and mostly sat in his shed. So I'm not sure how the maintence was. I would assume it would be well if it was raced.
So I am understanding as long as I am not strangling the bike every ride I could get some good use before another rebuild? How often are the valves checked?
Do you have any other mod tips/suggestions to promote a longer living bike?