ive only done a hand full of repaints so far but as for paints the water based acrylic stuff from games workshop has yet to fail me... might take a couple of coats to cover any details that are already there.. but its fairly cheap and comes in plenty of colours.
alot of people use testors or tamiya but i dont have much experience with either or these..
as for tips.. make sure your brushes are clean and well looked after.. try and plan or draw out what you wanna repaint him as or with what colours.. as for the decpticon logo id say reprolabels is your best bet.. saves alot of hassle trying to make your own stickers or transfers..
most importantly, if you get stuck or have any other questions.. just ask! thats what these forums are here for!
