Ooooof.
Well. If you have a C&R and a Gunboards membership, that's where you'll find the most info and probably the best deals.
What I can tell you is that when I was collecting SMLEs back in the late '90s, the going rate was about $250 for a really good one. Now, I'd think it's closer to around $600 for something comparable. There are MANY things to educate yourself about, and Gunboards can help you:
All-matching .303 SMLEs can be found made by British, Australian, and Indian arsenals. The British stopped making them when they started making the No 4, but they were produced by the Aussies (at Lithgow) until the SLR was adopted, and by the Indians into the 1960s. Generally, SMLE folks frown on Indian SMLEs manufactured after the British stopped running the factories, at which point QC became iffy.
Now, the .308 rifles were not conversions. They are Ishapore 2A1 rifles. They were what the Indians used in reserve after the .303, and were built from the ground up for 7.62 NATO. They're generally very good rifles for what they are, though they do often look like dog doo. Prices on these used to be under $100, but that was (again) during the late '90s. I suspect you can find good ones for under $500 today.
Non-matching SMLEs, known in the community as "bitsers," were usually assembled from leftover Lithgow parts. A New Yorker named John Jovino bought up all of Lithgow's unused parts when the Aussies stopped making the SMLE and assembled them as non-matching rifles. They're often good-looking guns that shoot well, though there's nothing historical about them. Think, maybe, half the value of a genuine matching one.
As far as use, apart from the SMLE's ubiquity during WWI, it served the British until Dunkirk and the Aussies all through WWII. Most of the British in the CBI theatre ended up with one also, since the British controlled the Indian arsenals at the time and that's all the Indians were tooled up to make.
You can look around yourself to figure out the niceties of volley sights, Weedon repairs, draws, precautions while disassembling (pay attention to the king screw and watch those handguard ears!), and magazine cutoffs. Researching these things is half the fun.