Should I use a stick with a wooden tip? Or plastic? How heavy or light?

These are all very personal questions and you really do need to buy many different kinds and experiment.

A plastic tip will give you a crisper sound on the cymbals compared to the wooden tip. If you are playing a softer, quieter song, maybe wooden tips aid the sound of the song better. Wanting to articulate some fancy high-hat work, plastic tips will give you that extra bounce for the one-handed rolls.

For stick sizes, the numbers go up for the thinner/lighter. 7a is thinner/light than a 5a. In marching band, I used 2's. Use the right tool for the job. Now that I play almost solely on a church worship team, I use 7A.

"Should buy a pair of mallets for the odd cymbal roll?"

No. Buy some sports wrap and wrap it around the butt of your stick. Make a small ball, enough that is is spongy and you cannot easily feel the wood underneath. Now, when you want to do a nice cymbal roll concert-band style, just flip the sticks around and roll away! You might want to practice the stick flip maneuver a few times. You just flick your wrist as you open your fingers so the stick can rotate on the palm of you hand.

Boom! You're rollin'. Boom! You back to normal playing.

So, get out to your local music shop and buy a little of every stick. Find what suits you. Pay attention to grip! Sticks can become slippery when your hand get sweaty, thus I prefer plain wood finishes. Painted sticks look cool, but they not cool when they fly out of my hand.

One final note...buy a stick bag. They hang conveniently off the side of the floor tom. Always have multiple sticks at the ready. I cannot count the number of times I've lost a stick as I've lifted it from the snare, hit the bottom of the high-hat and lost my grip. Air Drumming!