A $200 system isn't really feasible unless you get an old used system or parts being thrown out by a college or other institution; you can cobble parts together to fit that bill.
I wouldn't trust it. I use old systems for testing, for one-off things that aren't important tasks, etc...you'd be talking about slow systems with parts that are either uber-cheap or old and thus could fail without a good backup in place, so I hope the data isn't too important.
If you're talking about a server that is actually used for business, i.e., you have money riding on the data or will experience difficulty if it crashes or loses information, you seriously need to reconsider the amount of money you're budgeting for it or consider outsourcing...and it sounds like you need something reliable if you're going to use it as a PBX.
It's true that some old systems will run and run and run, but if I had money riding on keeping that system going, I would have money allocated for the system, a backup system of some sort for proper data recovery, and a backup computer for replacing the loaded system should it (when it will) fail.
You also might want to look at finding a way to separate your services a bit if for nothing else than to simplify management. You're saying you want email, web serving, PBX, and a file server, along with antivirus and backup on it? I've had cases where program interactions can cause issues (especially antivirus)...something to consider while planning. Also upgrading and if you decide to change your platform of choice for certain functions (like CRM products) can cause issues. Is it going to be an issue if your email platform needs to be changed or upgraded and it interferes with your web server, or if in the course of fixing email you take down all the other services?
One solution for that, especially if it really is lightly loaded, is VMWare or some other virtual solution. I'm not too happy with VMWare now that they're bullying other vendors to cut functionality from their products out of fear of their own business model being threatened (thus creating bigger opportunity for Citrix, Xen, KVM and Microsoft's Hyper-V) but I mention them because we're currently running 7 servers of light activity on a single Poweredge 2950 from Dell without issue so far. Here's an article from a guy who made a rather beefy server for $600 to run ESXi (which is free) and then you can create many small servers for each purpose and thus test, upgrade, and swap out platforms as needed (and if you get a better server later, install your VM platform of choice and just copy the virtual machines over with minimal hassle to your software installation and still get better performance!). The only thing that may be an issue is the PCI card for your PBX.
Please...reconsider the budget. Being budget conscious is one thing, but it will cost you more in aggravation and headache (and repair costs!) down the road if you go too far with saving money; if this is an important part of the business the budget should reflect that. Please please please, as someone who has seen others make the same mistake and pay for it later on...