Choosing a sewing machine
Details on how to find and choose a sewing machine for automotive work.
 How it works
Sewing machines basically join fabric (cloth, naugahyde, vinyl, etc.) by punching a needle through the material and knotting a thread to create a stitch. The issue is that sewing machines aren't subject to the equal rights amendment, and some just don't cut the mustard.
Ever since Mr. Singer figured it out, nearly every sewing machine works the same way. A thread is carried by a needle that pierces the fabric from the top. As the needle pokes through the fabric, a rotating bobbin [shuttle?] below the material catches the thread and magically knots it to a second thread carried in the bobbin. As the needle moves down vertically and pierces the fabric, the "foot" holds the fabric down for the stitch; when the needle moves up, the foot releases and the "feed dog" below the fabric pulls the fabric into position for the next stitch.
The useful controls on most machines for our purposes control the stitch length (i.e. number of stitches per inch) and tension of the thread(s); normally a foot pedal (potentiometer) controls the speed of the machine. Most machines (home and industrial) will have a backstitch control, which enables you to overlap a few stitches at the end of a row by literally running the machine backwards. This ensures the row of stitches will not unravel.
 Your wife's (sewing machine is) a problem
Unfortunately, most home sewing machines are not adequate for an automotive upholstery job, although it can be done, but not well. Most home machines - Singer, Kenmore, etc. - can drive a needle through two or three layers of automotive fabric, but they don't have the horsepower to create a firm enough stitch in a suitable weight thread for our purposes. The result is that the seam will not be tight, and it will pull apart when under tension, like when your girlfriend sits on it. The typical beaded seam involves at least four layers of material, which is beyond the capacity of your mother-in-law's machine. If your wife has a sewing machine, find a new wife. In addition, padded seat or door panel fabrics may be too thick for your average machine. Tuck-and-roll upholstery and door panels have the thickness issue, plus are multiple layers thick. You could probably jam the material through a home machine, but it's not going to last. That's the issue.
 The solution is downtown, or by the harbor
The solution to trying to make do with a home machines is getting your hands on a used industrial machine, like an old Singer 111-W. Most cities worth the name have the remains of a garment district with dusty places that sell and service sewing machines, and I don't mean your strip mall Sears, or the Singer shop. Sailmaker supply shops are other places to look. I've seen used heavy-duty machines for $250 in New York, and there are places that rent machines. A sewing machine is a terrific addition to your man cave, and I bet you could get your (new) wife to agree to buying one a heck of a lot easier than it was getting her to sign on to the plasma cutter that you might use once again in your life.
 What to look for in a heavy-duty sewing machine
What separates a heavy duty industrial sewing machine is raw power and some other features that permit precise and strong sewing of heavy fabric, such as upholstery, leather, canvas, etc. The industrial machines normally are belt driven, with a separate electric motor, whereas a home machine normally has a puny built-in motor about as powerful as an aquarium air pump.
A feature to look for in a man's sewing machine is what is called a "walking foot," which is when the foot that sits on top of the fabric acts in unison with the feed dog below to pull the fabric into the machine and position it for the next stitch. This practically eliminates fabric slippage, which is a problem with slippery automotive materials, and it helps ensure an evenly spaced stitch.
A further refinement is a "needle feed," in which the needle actually acts in unison with the walking foot to pull the fabric forward, resulting in very precisely spaced stitches.
A "double needle feed" machine is the ultimate, in that it has a needle above and below the fabric.
Any of these industrial strength machines will require a separate bench to mount the machine, its motor, and the foot control.
 Sewing isn't for wussies- some words on safety
These heavy-duty sewing machines are dangerous and need to be treated with respect. These machines can't exactly tell the difference between running a row of stitches across a seat panel or up your arm, which they will do. A double needle feed walking foot machine will suck your hand in and stitch it right to your tuck-and-roll door panel faster than you can say "waaa...?" So be careful. Try to stick a needle through your leather belt and you'll get some idea how powerful these machines are.