search ggogle and ebay for a5 shrouds if realistic looking a5 is what your after, can have it looking like a fair few real life firearms such as ak47, ump, g36, m249, barrett M107 to name a few ive seen
thou you'll need to remember the more realistic your wannting it to look if you go down the shroud lane then more bits on the marker mean more weight, i had a ump shroud on my a5 was nice but a pain to clean and jus more weighty, now i just use a G36 folding stock, one point sling and j&j barrel kit jus ditched the ump shroud to local site as found it to be a bit of a pain in longer games
as for apex/flatline its down to personal preference ive both and have used the flatline more tbh thou personally have tried the apex properly part from trying the curve shots
hope thats some useful info bud