I bought my SE, with just under 60k miles on it, knowing that it needed a 20k mile service. That enabled me to buy it at a reasonable price.
I drove the bike for a few thousand miles, having only done engine oil & filter, as well as final drive oil changes.
The following winter I did the 20k service myself.
I bought a Triumph "service kit" on Ebay for about $150. Expecting to need to adjust the valve clearances, I bought a generic "valve shims kit"($50). Engine oil & filter cost another $50. Since my cam chain was making a little noise at hot start ups, I also bought the triumph service tools to set cam timing($100) & the replacement fasteners for the cam sprokets($20). DOT 4 brake fluid is needed for the brake & clutch line flush($15). I already had the computer based DealerTool, needed to flush the brake lines. A service manual is also recommended.
In the end, I had to adjust 11 out of 12 valves. I really don't think that they had ever been adjusted before, just measured & left on the"tight" side of clearances.(however, my bike did get a new head, as part of that recall, at about 20k miles).
So, the 60k mile service cost me about $400 & about a week of my spare time to perform, but I did already have the service manual & DealerTool system.
Results: valve clearances are in good spec & cam chain operations are quieter. The engine runs smoother. I am much more familiar with a lot more of my motorcycle & am more confident of it's operations & condition(I did buy a 60k mile used motorcycle!).