As waitronunit said, you may end up spending alot of money on converting micros units or any other hardware,but yes it can be done. I have worked for a couple of different dealers and if you don't buy a turn-key system they will get you good with charges.
I have seen customers sub contract their network wiring and any and all network problems will be blamed on that contractor and the POS company will charge you every time they have to come out and work on something they did not do or sell you. Many dealers give a 1 year warranty on hardware and software out of the gate. Whereas with your used equipment there will be charges for repair, travel, and possible loaner rental. Use your best judgement, and weigh the pros and cons of used versus new.
One example that comes to mind is some of our customers want to provide their own network server because they see they can get a Dell to our specs for 7-900 dollars instead of pay us 1600 dollars for ours. Now they have bought the Dell and have to bring it to our office so we can program it (load software,setup networking etc.). We charge them hourly to do this and yes they saved a couple of hundred dollars but now when the server crashes, and all computers do, They have to call Dell for warranty or pay us hourly to service it.
Good luck finding a used Aloha software key. If not your loacal dealer will sell it to you, I'm not a sales rep, but I beleive the license are around 1000 dollars a terminal. Also, I have heard of Aloha giving price breaks on replacing systems like Micros and Squirrel, so you may have some trade in value. Again check with your local dealer on that.