I am pretty new to painting myself but will go ahead and post some things which have been helpful to me so far.
First, you definitely want to use primer all over your mini. If you use paint as the first layer, it will just start peeling off whereas primer will stick to the mini and the paint will stick to the primer.
I prefer black primer. White can be a lot harder to completely cover up and can sometimes make a mini look too bright (for my tastes).
I prefer metal and metal will just look better even with a non-professional job whereas with plastic, it has a tendancy to look bad unless you are *really* good at painting. Metal is also sturdier and if you're going to go through all the time and trouble to paint something, paint something of quality that you're going to want to keep and use. Nothing wrong with using plastic of course, but if you like the feel of metal minis, then just go with metal. If you end up making huge painting mistakes, you can either paint over the mistakes or you can just strip the paint off and do it again. It's easier to strip paint off metal than plastic.
I use brush on sealer nowadays because the spray dullcoat sealer came very close to messing up my minis. It was extremely cloudy. I haven't heard anyone else reporting this problem (I was using Citadel sealant and had shaken it briskly for over two minutes before use), but even with using light coats, I ended up having to use a thinner to strip off the sealant and redo. Thank the gods the thinner worked to salvage them and didn't destroy the paint job or those minis would have needed to be stripped and redone completely. Now I use brush on (two coats of gloss and one dull with gloss touchups afterwards where necessary) and the sealer dries as clear as crystal. If for some reason you have trouble with the spray sealer, then brush on sealer might work better for you. It did for me. Either way, I apply one thin coating every 24 hours for three days.
As far as paints go, I like the Citadel brand paints, but find myself using a mixture of Citadel and Anita's all-purpose acrylics from...hmm some craft store like Michaels, but it was another name I can't seem to remember. Since the Citadel paints have these really crappy lids that get stuck, I can't get into most of them without using a sledgehammer on them, so I use the ones I can use and use the other paints for everything else. I refuse to buy more Citadel paints until my gaming store gets the newer ones which have flip-top lids. The Citadel paint itself is great though--very smooth. I haven't found the need to thin the paints too much for normal painting but will keep the brush a bit damp (not wet...use a paper towel or something to soak up the excess) at all times and just make sure there's not too much paint on the brush.