You made no mention of the previous owner replaing anything, so here's the bad news.
On a 4 year old bike that was ridden consistently it's likely due for new valves, new valve springs, valve seals, a valve job, piston and rings, a cam chain tensioner, waterpump shaft, water pump bearings and the shaft seals. These are all standard high wear items on the CRF 450s.
Other things to look for are wear on the exhaust rocker roller, rusted steering head bearings (very common), and the common high wear items like linkage bearings and wheel bearings.
Changing oil every couple of rides is common, and a good practice. The sump is very small on these engines and as a result they hold very little oil, so changing it regularly is a very good idea. Checking the oil level every ride is important as well.
It's a great bike, that you are bound to really enjoy.