hi
it is difficult to answser this qs but i will do my best coz i have good experience
first of all do not worry about registration, it is so simple after to choose car go with seller to the ...eg Madinat Khalifa Traffic where it take about one hr and cost 100 QR.
then to get used car i advice u to have look in many different places to get idea about prices for example
...... websites
...... industrial area street number 10 specially in weekends
...... salwa road where are many car showrooms
....... bulletin boards in almeera
....... etc
after that or before of all i said choose which car (i mean which company and model) you want buy coz if u go blindly u will get lost and u will waste alot of time to choose a car.
after to get idea about prices choose the car u want to buy from any where after to come to agreement about the price with seller then take it to computer checkup in salwa road( cost about 200 QR) where they will tell u about mechanical details of the car which will determine to complete the deal or not.
for more detail ask again