If your a small dude, the fx and rt mod armour will drown you, they are made for the taller bigger trooper in mind.
Someone of your stature will better suit the more screen accurate sized armour out there but i think you will still need to modify this so that you can tackle simple trooping tasks such as stairs and walking.
Typically you will need to trim down the legs and maybe even arms to be able to move easily. When the other guys here say do some reading they are not being rude, most of the info can be easily found here to answer your Q?s.
I would still advise you to take some time to have a read through the various build threads and get an idea of how things go together. And also take into consideration that not only will your body size effect what kit you should buy, accuracy, price and difficulty of assembly should also be looked into.
As a rule i would advise getting new armour so you can easily tailor make it to fit you, buying second hand is not always the easiest path...