Corsa's are an easy car to install in, a chap covered all the fundamental issues are noted on the previous page (door magnet hole size for the woofer, mounting depths etc) from a brief scout through, note down all of his advice for install day
Do yourself a favour and stick to a 5.25" woofer and you can basically ignore any issues

I've still got a Corsa myself with a bit of ICE in.
Depending on the components you go for and mounting hardware supplied with the tweeters you can quite easily put them in their standard place at the top of the doors by the door handles. Mine are actually mounted on the door mirror covers higher up for a bit sweeter sound to me.
You'll need to hide crossovers for components, but behind the doorcard underneath the armrest with a sticky pad is fine for units supplied with those Hertz Dieci comps I recommended. Small and thus easy to hide

(EDIT just noticed this was mentioned in the form of cutting the foam away from behind the doorcard too, to hide them in there if you wanted for bigger crossovers).
Amp/sub/wiring deals, easily done for 100 quid. As said, road noise will kill bass... you will hardly notice any bass from your comps in all honesty once moving. Also, I'd recommend you run them with a high pass filter at 80Hz, allowing you to go louder for longer from head unit power and then letting the sub do bass 80Hz and under.
Do yourself a favour, don't go FLI! lol, they're honestly terrible subs. And unreliable amps built into them.
I'd recommend a cheap JBL deal for sub/amp/wiring/box. Just screw the amp onto the back of the sub box and you're done. Wiring is easy enough, some torx screws along the cills of the car interior to undo and route cables under there remembering to keep the phono cables on the drivers side to ensure you won't get any interference from power cables, which are best ran down the passenger side as theres a big gromet under the glovebox you can poke through into the engine bay and over up to the battery, most direct route... run the remote cable with the power cable (remote to back of head unit to switch amp on and off with the HU power) and then earth cable from amp to back seat mounts or a suitable earth point.
Sounds complex but a few hours careful work and you're done
Also, don't spend loads on front components if you aint amping them. I don't think you'd be doing the Diamond Audio ones any justice running from head unit power... Some nice quality Pioneer comps available for around 50 quid too which run off head unit power pretty well but the tweeters are big and cumbersome and awkward to mount.. Think the model is something like A130Ci or something, maybe worth a nosey though.
If you wanted to avoid sub and amps, maybe some Hertz Dieci 6x9's and a stealth shelf would do the job? All depends on budgets, but the dieci range are specifically designed to be run from head unit power as they're very efficient speakers and can make plenty of noise without much power but still sound pretty nice for the money!