I sewed for many years on a 1950's era straight stitch Singer 301. That machine could handle all sorts of abuse, but it couldn't zigzag, and the buttonholes were so painful that I preferred to do them by hand. Or eliminate them by any other method possible. I bought the equivalent of about a $100 machine today... a mechanical that did zigzag and blindhem and a few other useful stitches. That machine sounded like a bucket of bolts falling off the back of a truck, and never did stitch very well. In fact, I found that I really disliked having to sew on a machine that had every symptom of hating me. When the main gear decided to split while I was hemming jeans, I nearly cried with relief. Completed garments from that machine were about $10 each, dividing the cost of the machine by the number of garments. In addition, you cannot get repair parts for many of the cheapest machines -- they are considered disposable.
I was going to buy another mechanical machine, as I was worried the electronic machines would be unrepairable as parts became obsolete. The family engineer persuaded me to at least try some electronic machines, and I fell in love with the precision and the fact that you get full needle punching force even at very slow speeds, something a mechanical machine with a standard motor cannot do. I wound up buying what is now equivalent to the Viking Emerald series, and it's been a great machine for the last 15+ years. Amortized cost of the machine for completed garments and quilts is less than $1.
Machine lust hit again this winter when I saw the buttonholes made by a Juki F-600 and one followed me home... it's even more precise and more versatile than the Viking.
I'd advise you to find the best sewing machine dealers around and see what they can offer in your price range, new and used. While you're trying out machines, ask them to demonstrate buttonholes -- bad machines do not make good buttonholes -- and bring along scraps from your t-shirts and any other sorts of fabric you might want to work with in the future, and try. Also bring a fine Sharpie so you can write the machine make and model on the samples. Just in the process of trying machines, you're going to learn a lot.
You may also find someplace that offers sewing lessons on their machines... that's another good way to learn what you like and dislike in a machine.
Of the new, lower end machines right now, my vote goes to Janomes and Jukis as the most for your money. The beginners I've helped with Janomes have had far fewer problems learning their machines than the ones with Brothers and Singers. Juki is a Japanese manufacturer that doesn't advertise much in this country, but I've sewn with their basic mechanical, and it was a pretty solid little machine. Janome is another Japanese manufacturer; they do a lot of machines for a lot of other companies -- Kenmore is Janome in disguise now, and so, I think, are the Husky series of Vikings. I'm not sure who makes the Bernette line for Bernina -- I think it's Juki (they do Bernina's sergers), but it may be Janome.
Don't evaluate a machine on the basis of stitches per dollar spent; the stitches you'll use most are straight stitch, zigzag (4 mm width is fine, more than that is gravy), blindhem and stretch blindhem (learn how! it'll
save you hours and hours!), maybe some triple zigzag (=tricot stitch), which looks like big zigzags with each zig and zag made of three small straight stitches, and some form of buttonhole that doesn't drive you nuts. Freehanding buttonholes with a zigzag sewing machine can be done, but it's a lot easier if there's some sort of automation -- 1 step or 2 step or 4 step. I would not care much whether or not it's got an automatic needle threader -- you can't use them on smaller size needles, and they're usually the first thing to break -- but I would like to see adjustable presser foot pressure (which gets you out of some fabric feeding problems) and a variety of presser feet that are not going to cost you an arm and a leg. The old short shank feet are often very cheap, and many of the Japanese snap on feet are not all that costly. On the contrary, some of the European manufacturers' proprietary feet may cost an arm and a leg. <g>
Stay away from the "mini" machines -- they just don't have enough structure to stitch well or for long, and the ones that are "battery operated". The Janome Jem series are decent little machines if a compact, lightweight machine is a must for you.
If you were my neighbor and wanted a new machine, inexpensive but good, these are the sorts we'd probably look at first:http://content.janome.com/index.cfm/Machines/Retired/Magnolia_7312http://content.janome.com/index.cfm/Machines/Sewing-Quilting/Magnolia_7318http://www.jukihome.com/products/hzl_27z.html
If you wanted a used machine, we'd probably be looking for Bernina, Elna, Janome, Juki, Pfaff, Viking (in alphabetical order), and Singer before 1970.
Try some machines above your price point, too... that will help you decide if you want the new basic Chevy or the used Rolls Royce.
If I had to pick a machine that would still be sewing in the 22nd century, I'd probably pick an old Singer or Elna straight stitcher. However, they are not terribly user friendly for beginners -- you need to remember to thread with the thread uptake lever (the nodding donkey thing) all the way up, and they have some strong opinions about the way things have to be done. I've let beginners work on my electronic machines while I've untangled whatever mess they've produced, and every one of them has found the electronics easy to work on. I won't let them touch my old Singer 15-91 unless they've sewed on older machines before -- it's just too persnickety to turn them loose on with no instruction. The downside of the electronic and computerized machines are that most will have a lifespan of under 25 years, and the required (supposedly yearly) adjustments are about $80 around me, while servicing mechanical machines runs about $50.
If you had only $20 to spend, we'd be hitting the garage sales for a Singer 15, 66, 99 or 201; I can usually find them for about $10 or so that way. They are vastly overbuilt, and if they haven't rusted solid, you can fix them yourself with the help of the yahoo group "wefixit" or others.