i will probably use a road hub, and because im using a road hub, I can use a road cassette, and therefore a short cage mech.
You can use MTB cassettes on road hubs and vice versa. My Alivio RD definitely works on the 9-speed 11-32 cassette, which definitely works on a 105 or any other cheap Shimano road freehub, so you could have your cranks, chainring, bash guard, BB, cassette and RD all sorted for under £50 by shopping in my parts bin.
You can use MTB cassettes on road hubs and vice versa. My Alivio RD definitely works on the 9-speed 11-32 cassette, which definitely works on a 105 or any other cheap Shimano road freehub, so you could have your cranks, chainring, bash guard, BB, cassette and RD all sorted for under £50 by shopping in my parts bin.