My '91 300E 4Matic w/ 191K is at an independent shop for a head gasket replacement for a very reasonable $950. They say the head is NOT warped

During the work, the shop called and said the plug wires had split and the car needed a cap, wires, and plugs for $370 including labor. I authorized the extra work with hesitation.

Then the shop calls again and says the car needs a new radiator ($268--they are going to be "nice" and not charge me labor on the radiator install). They claim that only half of the radiator is working properly. Is that complete BS, or possible? If they don't install the radiator, the shop will not warranty the head gasket work. I said let me sleep on it. Are they just trying to avoid cleaning the oil sludge out of the radiator?

I have sourced a new radiator just over $100--is this something that's easy to replace? I am about a 3 or 4 out of 10 in terms of mechanical skills, with 10 being a real mechanic.

Looking for opinions on my situation, please comment if you have time...

-GH