First of all it depends on your height, weight and proportions which armor to consider.
AM and RT-MOD are for the bigger trooper, AP is more for the smaller build (but can be modified for bigger troopers)
The rest of the armor fits the "average" trooper, 1,75m -1,85m, 70-90kg I would say (but this is no law, there is always room for modifications).
Then it comes to your budget - what are you willing to pay? The ATA (affordable trooper armor) set costs around $600, TM and RS are a decent lot more, but beeing more accurate.
And the assembly... well, this is no rocket sience, there are so many builds and tutorials here on FISD and we as a community will help you with any question you have. I would say that everybody can do this. The pre build armor is offered by TM and RS, but comes at a higher price, for RS around 2k.
I found it very satisfactory to build my own armor, something to be proud of, although I understand those who are not able or willing to do this.
In the end it is up to you, donÂ´t hesitate to ask questions ;-)
p.s.: very good thread: http://www.whitearmor.net/forum/topic/11538-the-various-types-of-armor-and-where-to-find-them/