Stalk adaptor is a dash-out job, there is a guide in the guides section for removing a PH2 dash - it's not difficult, but it is time consuming, will involve shouting/swearing but be sensible (patience, follow guide carefully, don't loose torx nuts, start in the morning leaving yourself lots of day light, don't just pull at trim till it snaps etc) and it's fine.
headunits are very easy, especially on the PH2 clio which uses a standard ISO plug. A pair of relatively standard "audio removal keys" from Halfords (£6 guess?) or Amazon (£1.99 with free P&P lol...) makes it an absolute doddle. If you can't wait/cheapskate then you can try tiny flat head screw drivers/bent coathangers etc first. Some cars theat'll work, others will be a right pain and will need the proper release keys.
Sub again isn't hard but be patient & do it properly. Grounding the ground isn't hard. Pushing wire through the firewall isn't hard but takes time. Routing cables under/behind trim/carpet isn't hard but takes time. Setting it up for best sound quality isn't hard but takes time (some good guides on the internet) Hardest bit sounds like working out WTF you've bought from ebay! Suggest you google the make/model and look for a PDF version of the manual/installation guide.
Definitely have a crack at it yourself before spending £50. If you spend £50 you'll feel a bit silly when you watch how simple it is...
