Refurbished US Airframes
There should be no problem with the Rivet Joint. Although the last C-135 airframe was built in FY 1963/1964, the USAF is anticipating not retiring the last until 2040 ! I can't speak for the mission equipment but at least it will have commonality with US variants and presumably upgrades will be offered from time to time.
The last time we procured surplus, refurbished aircraft from the US, we got the bargain of the 20th century. Qty 15 x F-4J(UK) were bought for only GBP 120 M including GSE and spares. Originally, they were going to be retired when due a Major servicing (after about 3-4 years) but in the end gave good service for 8 years (1984-1992) with 74 Sqn (Tigers).