Tons of patents are modifications of other devices/inventions. It is very rare to have a truly new invention. Without knowing the invention (and I do not advise you to disclose it to the coffee room), I can’t really tell you if it is obvious or not. The one nice thing about obviousness is hindsight is impermissible. Yes, it may be obvious after you see it, but the point is that it has to be obivious at the time of the invention given the state of the art at that time. Also, you can use secondary considerations to argue against a finding of obviousness: things like the (1) The invention’s commercial success, (2) Long felt but unresolved needs, and (3) The failure of others might apply in this case.
My recommendation would be to go to an attorney or patent agent. The difference between an attorney and an agent is that an attorney has a law degree. A patent agent is someone who took the patent bar and can prosecute patents before the USPTO. They are functionally equivalent and there is no distinction when you are prosecuting (ie getting) a patent. The only difference is the agent cannot represent you in litigation (ie in a court). Since an agent does not have a law degree, he is probably going to be cheaper.