Well, it's a matter more about personal interpretations I think. One could arguably say the Soviet Voskhod capsule was basically a customized Vostok capsule to fit 1-2 additional crew members. Booster upgrade could potentially be a separate component. It's hard to tell though since the upcoming customization feature is rather obscure.
I'd imagine at all worlds that lacks an atmosphere would probably use a modified lander model based on the lunar counterpart. To meet certain requirements, heavy radiation shielding could for example be installed for Io and Europa, which presumably would be heavy and hence perhaps require a more powerful booster. Alternatively maybe a strong energy shield could be installed instead which would require a heavy power supply, leaving it up to the player to choose how to approach his problems.
Worlds with an atmosphere like Mars, Venus and Titan could use a more aerodynamic lander akin to the proposed
Mars Excursion Module. The Mars lander would depend on a retro rocket for soft landing due to the thin martian atmosphere. The Venus lander would be heavily plated and require a powerful cooling system. For Titan it would require a strong heating system instead, since that place is really cold. I also think it should float incase if it would splash onto one of Titan's hydrocarbon lakes.
Perhaps customizing spacecraft could be slightly like when tinkering with power armors in Modular Powersuits? There seems to be a lot of possibilities, if maybe not a bit too many.
