It looks like you can do everything you want to do on a Mac. To read and edit your documents (MS Office files) on a Mac, you’ll need to download OpenOffice (free) or buy Apple iWork ($50) or MS Office for Mac ($200 and up, unless you’re a student). Your pictures will all open natively. My old saved TurboTax files are PDFs, which will open on a Mac. TurboTax will also run on a Mac from either the CD version or the web-based version.
Since you just want Windows for comfort as opposed to there being an app that needs Windows to run, I wouldn’t spend money on a new version. Just use whatever version of XP you already have lying around. All your stuff going forward will be from your Mac so you really only care about backwards compatibility, not forwards compatibility. For security, there are a few good free suggestions in this thread, but you can just set up my Windows VM installation so that it can’t access the internet. You can do all your email and web browsing on the Mac, and manually move files back and forth if there is a need to do something in the Windows VM. Then you don’t have to worry about security at all.