Are dumbells enough for you to get muscles?

yes.
Scoobys workshop site is devoted to home bodybuilding and most of the exercise is with dumbbells.
lots of free workout videos and information on diet and cardio too.
best home bodybuilding site there is.

The problem with dumbbells is that you will grow out them relatively fast, not to say it can't be done because it can. It just might take more time. I suggest getting adjustable dumbbells as it may save you money in the long run plus they're great space savers.

Here's a workout you might like to try:
http://www.bodybuilding.com/fun/rudy6.htm

sure they are over time

yes

Honestly, You could probably gain some muscle but you would be limiting yourself. I would try and find a cheaper gym if you can