As a sole trader...
I use both PC and Mac. PC is my main office-based computer, while a MacBook is my laptop. I work a lot with one particular company who are mac-based, but when I work with others compatability has never been a real problem. Sometimes I lose a bit of formatting when moving a document between the two (Office on the PC and iWork on the Mac) but it's not a huge issue, and I've used OpenOffice on both to reduce that before now. You can - if you do want to pay (I'm quite tight) use MS Office on the Mac. Without Office Mac Excel used to be a problem but I now have iWork '08 and am just about to try Numbers out. Can't tell you if it's an Excel replacement, but if cost is an issue just use OpenOffice.
The company I work with who are mostly mac-based do have a couple of PCs networked (on Mac infrastructure) and have no issues sharing stuff.
In terms of video conferencing, if it's using Skype then the MacBook is pretty good - it's got a built in webcam and mic - but that's certainly not a reason to go down that particular route. iChat is good, and I've used it when working remotely, but it's not exactly a unique selling point. Check if the software you use for video conferencing is supported on the Mac.
Really, there are very few major differences any more... the Mac is easy to back up using Time Machine but a PC is easier to upgrade. A PC is generally cheaper but the Mac is often better performing (start-up times, in particular, are very fast in comparison). PC software can be incredibly expensive while Mac software can be cheaper (compare £55 for iWork with whatever Office now costs...). PC software is better known and doesn't have that learning curve... which isn't that steep, really.
Of course, with a Mac you can run parallels or Bootcamp and have a bit of both, but if all you're going to do is work on Windows on it you may as well use a PC.
I know that's not particularly helpful, but to summarise - it's entirely up to you. Your concerns are valid but are far less of a problem than they used to be and I've got on fine running a business split between Mac and PC, and could have run it as well and as easily on either one platform. Can you find someone to borrow a Mac from to do some of your regular tasks - a bit of word processing, doing a presentation in Keynote, that kind of thing?