Depending on the water temperature you might like to go for a 3mm tailor made elios - warm and flexible and you will need less weight - probably warm enough for Greece in the summer (depends on water temp...).
But you might still be growing...
However - the Elios are not too expensive and you could get it shipped to your place in Greece ?
www.eliossub.com //
http://www.eliossub.com/html/enl_news.php then check out the tailormade link at the top - all kinds of neoprene - all kinds of finishes etc - A custom fit will give you (arguably) better warmth than an off the peg suit unless the offthe peg suit happens to be perfect for you.
If you were a little chilly in a 3mm shortie I bet you would be toasty in a 3mm freediving suit.
Hood...proper fit....etc = warm.
the guys at elios are friendly and will be able to advise and sort out what you need and give you a quote that you can compare to a shop suit. For example a mate has just bought an olive green 5mm (our water temp is 8C to 12C for most of the year and over the summer peaks at 18C - I would switch to a 3mm at about 14C for shorter length dives or at 16C and above for longer dive sessions) suit with kneepads - opencell inner and olive green nylon outer - pretty good for spearing and a perfect fit...and alot cheaper than an off the peg spearing suit locally.
Personally i think a 5mm would be overkill and you might get frustrated with the amount of lead you had to lug around. But I am guessing that the water is above 14C ?
About the gns - buy what you can afford - a nice gun is a nice gun
Any gun will catch fish (I have proved this to myself) - some are nicer than others. If you are into a nice looking length of wood and have the cash then why not !
If you are thinking of going many times then you may save money on the eventual upgrade - I bought an expensive wooden gun after using some of my mates aluminum guns for a while. I dont regret it and am going to buy another wooden gun or hopefully make one if I can work out how to have 34 hour days.
Wooden guns are addictive in a way that I dont find with metal mass produced guns - just my personal opinion... I would be tempted with a nice Totemsub Pelagos 90 (
http://www.totemsub.it/indexarbaen.htm) but that is my own personal recommendation and if you feel you should buy a metal gun my opinions should not sway you !
Get a knife too - this is as much for safety as for killing the fish - get a solid one that you feel comfortable enough to lose... Yes - you may drop it. If you get snagged in a line underwater you can cut the line, this happened to me a day after I bought a knife, and now I use a knife whenever I go even if only diving and not spearing.
I hope that is helpful.