Hey Jose,
For basic one color or two color fades the Badger 150/200 series airbrushes and their clones (sold at Harbor Freight) work just fine. They tend to have a medium taper needle (.5mm) & noozle which will spray from about pencil line thickness to a 2"-3" cone when held away. Those are a dual action siphon feed type airbrush which means you can attach larger bottles to them for longer spraying sessions. As for PSI, siphon feed generally need around 20-22 psi for thin liquid like water/spirit based dye as a starting point for a heavy coat and for top coat, generaly coverage 30 or so is good. No more than 40 psi or so for thin stuff or you'll get a ton of overspray. If you plan to spray stains & top coats you'll want to either thin it (generally 50/50 is a good ratio, you're looking for the consistency of regular milk) and you may need to up the psi 2-5 depending on how it sprays, atomizes, your movement speed, etc.
I generally recommmend a dual action airbrush versus a single simply because you have more control. Shooting basic, single colors & a top coat, single action works fine. But when you want to add two color fades, stippling, background for distressed items, etc. a single action doesn't cut it as well. if you're just getting started, the HF airbrush (Badger clone) will run about $15. As you get more into and decide to try more things there are airbrushes from $15 to $300 depending on what you want to do. If you stick with the basics but want to run a set-up that is easier to maintain, pick up a couple of the HF airbrushes; one for dyes, one for top coats. They are much easier to clean that way especially if the solvents needed to clean are different (alcohol vs water vs paint thinner) as if you use the wrong one, some paints and top coats turn into globby messes and then take an extra hour to pull apart and clean completely.
Speaking of cleaning, pick up a set of airbrush cleaning brushes in various sizes. They are basically teeny-tiny pipe cleaners for getting the nozzle and needle channel really clean. Keep in mind, on the cheaper airbrushes the bushings that keep the material from blowing back can vary. Some are not rated for heavy solvents so you'll likely be limited to water & alcohol based products. Higher end airbrushes have tephlon paxking o-rings for higher solvent resistance and polished stainless steel needles for fine detail spraying.
Another good source for a starter set is Airbrush Depot & TCP Global on Amazon. They have a knock-off of the Iwata airbrush with a basic air compressor for $75 right now: http://www.amazon.co...=pd_sim_sbs_t_3 Similar to the set you can get at HF but with a slightly better designed airbrush.
Hope this helps,
Chris