You can probably find one at the big box stores for that amout right now. For a few bucks more you can get a matching set. Another source is Craigslist. Can find some good deals there.

Thanks, Nod. I know that there are several available in her budget, it's just a matter of which one to get...are there any brands or models that should definitely be avoided?

I'd recommend a whirlpool washer and dryer. Their wash system is pretty bullet proof. you can still buy a mid range washer for around $400.
If service was ever required they are easy to work on.

If you still are unsure of what to get, some really good resources are youtube and expotv. With youtube you can see many brands in action and expotv has people who give video reviews. You can also find many written reviews by typing the name of the make and model of a machine followed with the word review in an internet search. For example, "Kenmore Top Load washing machine reviews", without the quotes. Tons of links will pop up.

I hope this helps you and anyone else who finds themselves in this situation in the future. It helps me a lot when I need info about anything.