Hi Roko, Def worth paying a gestoria to do it all for you.
They will check if the car has any outstanding fines/bills (including road tax due - this should be paid up by the seller by law before selling it on). You and vendor go along to Gestor at agreed time, sit down and produce all the originals of ID, etc, etc and a 'Compraventa' (buy/sell) document is drawn up covering all the legalities, and the price it is being sold for.
Officially the vendor should pay for the transfer,(as it will cost around Eu150? depending on car in total - there is also a tax to pay when a car is sold). But if they will not even go half, see if they will take it out of the sale price... (I.e still getting them to pay for it).
Best to do all of above and do it legally. Although, if they don't they will still be 100% liable for the car, including any fines you might incur.
It is in the best interest of all to do it legally/properly.
(You will get a temporarly 60 day 'white paper' from the gestor whilst all the paperwork is going through at Trafico. You need this to take out your own insurance etc)
Good luck!