@ Sleepyelf.
A 12' long plastic board definitely has its segment. With varying size sails and boards manufactured from the 70'S /80'S /90'S /00'S or 10's, they all work. Sail size from 1.5m2 - 6m2. Heavy duty plastic board from 15 to 20kg ish, with a rotating retractable centre boards (for most) and small rear plastic fin, fibreglass masts and alloy booms, either dacron or dacron/mylar sails (i.e. cloth or cloth + plastic sail). Cheap, cheerful and accessible are an option.
Modern gear - wider and more expensive with a mono film sail (meaning one layer of plastic plus carbon fibre mast and boom in most instances) probably better to learn on, but.....
Wind strength, body weight and location, scenario:-
You're @ 55 kg. Then a Windsurfer One Design (366cm x 63 cm wide & 175 litre) or similar complete, would price from somewhere between $500 and $1,000 (used) and would work. Wind strength from 0 knots up.... it could be referred to as a "Glide" sensation, like a SUP or Kayak, "cuts through the water" not "pushing it" or trying to get on top of it by "Planning" in under 12 knots.
Modern gear, for a 150 litre short board (around 2.5m ish length x 70cm plus width), "might," or "could" be more cost effective. But, in non planning i.e. trying to push a big wide board through the water to get on top of it, may not be as efficient in winds under 12 knots as the cheap and cheerful plastic or alike is.
"Up to You" how and where you spend "Your Money".
Example of slightly lighter weight with mid 40 kgs (a 12' plastic board from the 80's ish period). Works well and glides (or good value sailing under 12 knots). Learn how to rig and get sail board control in the light stuff. Then add and embrace shortboard planning fun, buy a short board that's modern, light versatile and fun.
Cost v Return. 30 year old board + "New(2014)" 3.5m2 sail + second hand (alloy) boom, on water and good at the fun factor for under $800.
Is a retro board or modern board for you, who knows. It's as you've requested, Advice (opinion). Who should judge, nobody, which ever you choose. Welcome to the best addiction/passion of your life.