What sewing machine should I buy?

The best answer is to go to a sewing machine repair shop or dealer and buy her an older machine in good repair. Older machines have many more metal parts and are more than adequate for the purposes you describe. You should be able to get a nice older Singer for your money, complete with attachments, etc. Ask the dealer to help you select and do not get talked into buying one of the newer computerized machines that can embroider, etc. They are not nearly as useful and will cost much, much more. I sew all the time; two of my machines are Singers from the early sixties and the other is an older Kenmore.

If you want a new one, the http://www.amazon.com/gp/aw/d/B000JQM1DE is great. I make a lot of clothes and quilt. It's just fancy enough to make things easier (magically easy buttonholes!) but doesn't have a million useless extra functions.

I agree with Jenna. I've got two older portable machines, both around 20 years old. I got them for free. One's a Pfaff retired from a high school home ec. program, and the other a Kenmore that I picked out of someone's trash. A little lubrication and minor adjustment, and they work fine. They are probably quite a bit heavier than a new machine, especially one in the price range you're aiming for, which might be an issue if she plans to carry it around a lot.

There is an inoperative Singer Genie 353 in the attic. Is it worth repairing? Would her sewing life be easier with something more modern?

http://sewing.patternreview.com/cgi-bin/sewingclasses/board.pl?t=33864&pn=1 the Genie is a rather desirable machine.

I'd get some colors and paper, or clip art, or a friend. Make a coupon good for one sewing machine. Take her shopping. That's what I do when I want to buy someone something that I really don't know enough about, or because it would please them immensely to have a free pass to "go buy the whatever". I wouldn't go in totally ignorant; shop a few places and have a few talking points, "See hon, this one looked easy to set up, but I thought you might light fancier stitches, so I couldn't decide without your input ..."

Nthing Tilde -- choosing a sewing machine may not seem like a personal decision, but it surely is. I've been sewing over 50 years, on just 3 machines over that time. I love(d) the ones I chose myself; the one chosen for me (my mom's machine) was a pain. When I bought my current machine, I tried out about eight or nine machines (as in sat down and SEWED on them - straight and decorative stitches, buttonholes, etc.) and ended up with a machine I had initially rejected out of hand when the shop owner first suggested it. Give her a coupon and have fun shopping with her (well, be patient anyway). Also, as Old Geezer alludes to, just a heads-up that $150 is the VERY bottom of the price range for sewing machines. You can find them, but how 1) comfortable and 2) durable they are tend to be a lot harder to find in the low price ranges. Check Consumer Reports -- they have good information about the durability and ease of use of a wide range of brands. Maybe include the article with her coupon?

portable you say? my good friend (who just happens to be head tailor at the l.a. opera) carries her machine around in one of those 'stewardess bags' (thats what i'm going to call it ;) you know...the kind of small suitcase with an extendable handle and wheels (hers is smaller, height-wise, than most i've seen...just-right sewing machine size and it fits behind the drivers seat of her car)

I think "portable" might not mean what you think it means in terms of sewing machines. All of them are portable. Some of them are fitted into a stand or cabinet that is used just to hold the machine (and would be screwed in - like how you screw a camera onto a tripod, nothing custom). But most of them have a handle (all new ones likely do) and if not, they aren't that big to just pick up and move around. Then there is something like this - http://www.amazon.com/exec/obidos/ASIN/B002VY4WXE/metafilter-20/ref=nosim/. Do not buy it or anything like it, it will never work great even for small sewing projects. I think that, especially for your price range, a used machine from a sewing machine repair shop would be perfect.