Maybe you need two machines? When you say "heavy duty", do you mean long hours of sewing, or sewing through heavy materials? And by "mechanical or electric motor" are you thinking of a treadle or handcrank machine (most machines do have motors), or was it a typo for "servo or stepper motor vs. standard electric motor"?

Most home machines are not meant for long hours of sewing -- a couple of hours of needle time a day is fairly heavy duty for a home machine. Even though those of us who sew at home are often in the sewing room long hours, actual stitching time tends to be in fairly short bursts instead of seaming for hours at a stretch like a commercial or industrial machine might do.

If you're looking for hours and hours of sewing time, I'd suggest you might want to look at commercial machines intended for tailor and alteration shops, at "portable industrials" like the Juki TL98q, or at a standard industrial machine with separate motor and table (and oil bath usually).

Home machines are made for sewing mostly apparel weight fabrics, using "dressmaker" weights of thread. Bulling through thick upholstery fabrics and such on a regular basis or needing to use threads like upholstery nylon v69 is not a job for a home machine -- again, you need the proper type of commercial or industrial machine to handle this sort of load. I do keep an old Singer 15 around for the occasional bit of sewing I do that needs heavy thread - but it's because it's a $10 machine and if I ruin the tension, I can fix it myself. Can't do that with my Viking electronic.

Motor types:
Non-motorized (treadle or handcrank) machines are generally vintage straight stitchers. Parts can be difficult to find, but they're a lot of fun to keep going and to sew on.

Standard electric motor: tends to stall at low speeds and you don't get the full "punching force" of the needle at low speed. Generally found on the lower end new machines now, with mechanical linkages.

Stepper and Servo motors: give you full punching force at slow sewing speeds; generally found on electronic and computerized machines -- this sort of setup generally allows multiple needle positions, needle up/down controls, etc.

Please go talk to the best dealers around about your needs and try out some machines before making your decision. In general, new home sewing machines below about $300 US are not very maintainable and not of wonderful quality -- I sewed one into unrepairability in less than a couple of years of not very hard use, and it was never a fun machine to sew on. My Viking electronic was a little more than twice that new, and it's been a pleasure to sew on in the last 12+ years -- it just sews with no fuss and bother, and does what I want it to do precisely. If money is tight, I'd urge you to consider a good used sewing machine rather than a new one.

Take samples of the sorts of fabrics and threads you'll be using. Test the machine out on them. A good home machine will handle everything from silk chiffon to bull denim with aplomb. Industrial machines, however, tend to be more specialized... there are machines that are designed for light, slippery fabrics and machines that will sew 3/4" of leather -- but you're unlikely to find a machine that will do well with both.

Some starter reading for you, if you can find them at your library:
Gale Grigg Hazen: Owner's Guide to Sewing Machines, Sergers and Knitting Machines
John Giordano: The Sewing Machine Book
Carol Ahles: Fine Machine Sewing
(these three titles are mostly complementary rather than overlapping)

Web stuff:
http://www.cet.com/~pennys/faq/smfaq.htm
machine reviews at http://www.patternreview.com
http://www.threadsmagazine.com/item/4147/updated-the-latest-in-sewing-machines

Personally, I like Bernina, Janome, Pfaff, and Viking (alphabetical order) for current home sewing machines in terms of quality of stitching and support.

Talk to dealers, sew on machines, check reviews, check on service before choosing a new machine. Consider a garage sale mechanical that you can maintain yourself for heavy duty or dirty occasional projects... the folks at the yahoo group "wefixit" can help you learn to do a lot of the basic adjustments on a mechanical for yourself.

Here is a link with some history.