I'm aim for high accessibility all the time and can outline some of my methods I take to development a quite full on accessible site. It does take a lot longer to develop, but government agencies pay you nicely for going that extra distance (plus it should be law that government sites have to be accessible to everyone though I agree, it should be mandatory for every web developer, personal site, business, whatever).
As for the plugin idea, there's nothing that I know of (there could be such a thing though) but there is what is known as Assistive and Adaptive Technology, that most modern Operating Systems have built in (or you can download/install) which can provide the user with Screen Readers, Screen Magnification, Speech Recognition, etc. You should be able to access this in your Accessiblity Panel, turning it on to test your website would be a good thing to do.
There's many ways you can go about this, my most basic method is develop for a text browser, though I can have images, it's important that the alternative text reflects something relevant. So you should also grab a text browser for viewing the site, this will show clearly how your browser is displaying content that a screen reader would pick up this site.
As with many sites, the navigation is important, but more importantly is the navigation within the page. Always have quick links at the top so that people can skip to certain locations faster (especially the content), this is vitally important, just as most people don't want to wait for sites to load, others don't want to wait for their screen readers to reach the content they're after (though in some circumstances, they have too because of the lack of accessible sites and this is why it's important we cater for everyone).
Once developed for a text browser, you should make a seperate text version of this site, which is just a plain text file. This text version should help you know whether you've developed it as efficient as you can. Especially navigation and wording used. Try to not use action words like click here, etc. Say things like Vists this page, site, etc and always give a bit more information about what it does.
The reason for this is that in the text version, all links, in page navigation, etc should be converted to their URL/URI address. So if you've got click here. Then it would be click http://www.thissite.com/
in which there is no information as to what it does, and you're using an action word that is useless in text versions, and more specifically is talking about a mouse action, when those who don't operate a mouse, may use the enter key as their action or laser pointer, etc. So try keeping away from action words like that. This maybe a bit too much for you to start doing now, but take it into consideration, since I feel it's probably more important for you to get your site finished for your friends, than to take longer than what you expected.
Unfortunately, I've got to run but I'll try to finish more of the development methods off at a later time.