Painting steel doors is very straight forward. Personally, I spray whenever possible. The best advice I can come up with is;
1- Clean the door really well. For primed smooth steel entry doors I also lightly sand, remove the dust, then apply my own primer before painting.
2- Make sure the weather stripping is removed and left off for a couple of days after painting. It's a real bummer when a good paint job is mangled when a door sticks to the weather stripping.
3- Condition your paint. 99% of the time I'm applying 100% acrylic paint to a door so I use Floetrol.
Now, the staining part is a bit different. You mentioned several different materials and door types; steel doors, garage doors, vinyl doors, etc. All smooth surfaces are basically treated the same. As above clean and then prime. Vinyl will be the most difficult to prime, I recommend XIM Plastic and Vinyl primer.
Now you will need a background color that will work with the stain. I use the Old Masters Wood Graining bases. These are oil based pre-tinted paints that are colors for different stain colors. They work really well. Check out this page for more info on this product line; <a href="//www.painters-toys.com/wood-stains/wood-graining.html" target="_new">Wood Graining</a>. Each graining base works with a set of 4 different Old Masters Wiping Stains to achieve a specific look.
Now that the base color is on you will need some stain and a tool that will make the wood grain effect. There are several different types and sizes of graining tools on the market. I have several and all work well in a given situation. The rocker style is the easiest to use but a bit limited when working on small recessed areas. Graining on a smooth surface takes practice. Everyone fails on their first try. That's fine, the wet stain can be removed with some paint thinner and rags. Once you get used to doing it, graining really isn't that difficult. Plus, you can grain all kinds of different smooth surfaces; doors, floors, furniture.
Many doors already have a 3D wood grained effect in the door, their embossed. These are the easiest to do as you don't have to use a graining tool. You still need a colored background, eggshell sheen. Now instead of the wiping stain you can use a gel stain. Old Masters makes the best, I have tried a dozen different manufactures and Old Masters is by far the easiest to use and most forgiving.
My site gives a lot of tips on painting and staining different surfaces. Check out everything on doors for ideas. The best advice I can give is to practice on your own doors before giving bids to customers. This way you will be comfortable with the processes and you will have an idea of the time it will take.