We’re very happy with our 2008 C30 (T5 R-design, 6-speed manual). Bought about 7 years ago and have had only a few relatively minor issues (blocked sunroof drains, stone damage to A/C condenser, failed fuel pressure sensor, torn CV joint boot). Other than that it’s been just general maintenance, including the recent 10-year timing belt change. Fuel economy average is around 7-8 L/100 km since we’ve owned it (bought with around 80k kms and now has around 140k kms)
Main thing as always has it been maintained properly (good-quality oil, filter, timing belt, coolant changes, brake fluid changes, etc). I think the P1 (C30, S40, V50) represent good value on the used-car market, and if you want a manual the C30 is about the only option as there are very few other manual Volvos on the market at any given time.
Need to consider usage patterns - if kid seats required or frequent rear-seat passengers, I’d avoid the C30 just due to ease of access (or lack there of) due to the large doors (however it’s comfy back there once you’re seated - and C30 is only a 4-seater).
Hope that helps!