Okay, all my answers are going to be provided on the basis you're going to be playing on VAC protected servers.
1. About 1 in every how many players is a hacker in CS:S?
For me, I'd say about 1 in 50 players if not more. Most players are too scared of getting banned on either the server OR the game.
2. If I go onto a good server with a lot of players, how likely am I going to get super-rapid-fire-knife-nuked by a guy 1000 feet deep in the ground every time I spawn?
Not likely, most players will quit and change server if theres a hacker online with no admin to ban them.
3. Are hackers able to unban their accounts?
Nup. They would have to buy CS:S all over again if they want to hack on protected servers.
4. If I decide that I want to hack CS:S, is it going to be a pain in the ass with a 90% chance of ban or is it going to be so easy a baby could do it?
Oh, you will get banned eventually. Best to check if the hack you use no longer works for other people each time before you use it.
5. I know TF2 (Source engine multiplayer game also) has the occasional aimbot which gets banned so fast it makes people's head swim, is it the same for Counter Strike: Source?
Well most small servers have an admin online if your playing in their timezones peak hours. As for getting VAC banned, takes some time to process. I hacked for about 1 month, then stopped playing for 2 weeks to come back and see my account banned.
6. How many players are likely to have less visible hacks, such as chams and double-damage weapons?
Lol, most idiots go straight-out and turn on everything. Very few people such as myself are actually smart enough not to make every single shot a HS. Wall hacks are harder to detect but you can always get an admin to spec someone if they are on.
7. Again, am I going to buy the game to play a legit game just to get sniped by hackers from mars?
Nup. Play the servers that are run by groups, like Australia has internode or something that has around 20 of their own CS:S servers. Most hackers avoid these areas and go to smaller servers.
8. Would you recommend this game as easy hacking fun or a legit multiplayer shooter?
Legit shooter. Though if you want to hack, be aware that Steam will ban you from all VAC protected servers on ALL games. Eg. Hack on CS:S and get caught and you will be banned from HL2 VAC servers too.