things to do on a high mileage rover? how long do these motors last

Hello all,
i have a rr classic lwb 1993 have done the valves and head gaskets at 155.000 currently have 246.000 on the clock runs great, but with a few questions
Should i go ahead and replace timing chain and gears, also have a little seepage of oil in coolant put a block tester on the coolant res does not show exhaust in coolant( would change color if present) also no excessive pressure in system any ideas?? also what is the average mileage you can get on these engines.... I run amsoil and am on top of maintence cylinder readings are all 145-150 with one being 135 its no 8
Thanks