If you can aquire all the parts for cheaper than the cost of an actual fully built AEG than by all means do so. If you have walk access to walk in retailers who can sell you all the parts you need then do it up, but if you have to order all your parts seperately then in the end once you factor in shipping costs you maybe spending more than what your saving.
Assembly can be tricky if this is your first scratch build. You will require a good set of screw drivers are varying sizes mostly phillips, some needle nose pliers. If your considering scratch building, its best to try to keep everything to one company so you can avoid possible fitment issues. If you want to save a little bit of time you can purchase a fully assembled mechbox from whatever company you choose, that element saves you alot of time and effort since you dont have to purchase parts and assemble a mechbox.
Once you aquire everything, set yourself up on a clean work space with lots of light. Watch a few videos online from mechbox.com and those may be able to help shed a little more light on assembling an armalite. Honestly though, for me for 100 EUR difference I would rather just spend the extra to save the hassle and be guaranteed a working gun with no issues. Since you bought it in a shop, one would assume that if you have an issue you can return it for a refund or exchange.
"Its only a little bit on fire"