1) Has your server (if you own one) ever been attacked? How often?
yes,
2) Do you use any kind of DOS protection method?
yes, i use OVH as already said by nightwish this is a good, cheap protection. if you secure your server a lil bit, you should survive most of the attacks.
3) If yes, which one? Would you recommend it?
good question, if you don't have enough money to buy a better protected root i would recommend it. but if you have enough money to purchase a dedicated server from incloudibly, i would rather take the incloudibly server
4) Would you be interested in a low-cost DOS protection service?
maybe. for website hosting it would be nice but i prefer to host my gameserver myself, because i don't want that someone get's access to my data.