I was/am in the same situation as you are; my 73 TCS had sat for 10+ years before I bought it. I drove it only a few miles as I was intending a full restoration. Life and other stuff got in the way and I let it sit for another 10+ years. Because of registration/title issue i didn't attempt to drive it during this time. The car was represented by the seller as having a "recently rebuilt" engine but no paperwork to back that up so I could not be sure of the condition. I fired it up periodically then retreated to turning it over by hand.
Fast forward to 2 years ago when I decided it was time to either sell it (not likely because of a title/registration issue now solved) or actually start to work on it. I actively kept up with several web boards to increase my knowledge of the car and try to determine a way forward. Like you, I had read of the dreaded water pump and figured the extended period of non-running didn't bode well for the water pump seals. I also had the great fortune to meet and begin a relationship with Grumblebums (Joji) and a now-gone former board member Whit Davis. These guys were invaluable in hashing thru my sometimes crazy ideas of how to proceed. Fortunately, since I retired in 2014, time was on my side and I wasn't in any rush.
So, this is what I decided to do:
1. Accepting my mechanical limitations, I decided that removing the engine/transaxle was not that difficult and I could then find someone who could evaluate what condition it was in. This also assumed this "someone" could repair/replace whatever was needed (including the water pump). I'm budgeting between $4,000 - $8,000. for this. My engine is now at the rebuilder, along with some new parts I figured it would need.
2. With the engine out, I felt comfortable in attacking the rest of the car. So far I have replaced the radiator, all hoses, coolant transfer piping, seatbelts, front brake calipers, front suspension bushings, two front suspension arms, Anti roll bar, front shocks/springs, rear suspension bushings, rear shocks/springs, rear brake shoes/wheel cylinders, rear lower control arms, removed the original boosters, replaced the original MC with a NOS S2 MC, re-plumbed the brake piping, new shifter ball joint, shifter bushings and probably 20 other things. All of this was time consuming but not too difficult.
I realize not all the stuff I've done will work perfectly the first time I try to drive it but I've factored in testing time to chase down all the bugs.
My goal is to have a mechanically sound car to fool around with. At this point I'm not real concerned about appearance (the paint is terrible but no body damage) and I will likely tackle the interior after I get it running.
Like most older/classic car purchases, I probably would have been better off buying a car that had already been restored and that could have been registered but if I was that smart I wouldn't have considered a Lotus (although I did have a 1969 Type 54 S2 I drove for 5 years).
The above is just one data point on how I'm dealing with an almost 50 year old British sports car.
Larry Mullen