I have the exact same model as you on a 2000 plate. Same colour and almost same milage.
Sometimes the boot latch gets stuck out after you open it, I had the same problem on a W210 as well. On both cars plenty of WD40 has worked.
My C230k seems to have lazy central locking as well. It takes a couple of goes to open sometimes despite new fob batteries.
Not sure if either my issues could cause your fault, I've always been able to get the boot open.
In regards to value, I paid £795 for mine about 4 months ago. However I feel I got a very very good deal from someone who was desperate for the money. I believe I could make a profit on the car if I sold it now, how much I'm not sure though.
I plan to run it for as long as I can as a depreciation free car, I love it!!
Sorry I can't offer better answers.