As for Europa upgrades. I took another look at it. The Blue Streak 1st stage had a serious problem with it that would make substantial upgrades at best, extremely difficult. It was pressure stabalised with tank walls of 0.48mm thickness. This means that even small solid rockets on the sides could be too much for it, while larger solid and liquid boosters are a definate no. This is one serious issue that they'll have to face, and a major potential friction point for them.
It's actually pretty easy to fix: roll the stage out of thicker steel. They used to tailor Centaur tank wall thickness to the specific mission, changing it mission to mission to either add support or shave unneeded mass. It's not trvial, since it does effect how you carry out some of the manufacturing operations, but it's pretty easy to do without substantial re-design.