Three paragraphs is what I shoot for. Tell them who you are, how you found the position, why you're interested and what sparked your attention/what made you think the position was good for you in the first paragraph.
In the second paragraph, pick 2-3 requirements/expectations/qualifications...
I know you're not intentionally trying to stiff anyone, but that's quite a laundry list of things to do. It's worth significantly more than $75-150.
That could easily take 8+ hours.
If I were you, I would really look into getting cPanel or something like that. That will do everything...
The tests are usually given at community colleges, things like that. You can register for them at http://www.pearsonvue.com.
If you want physical equipment, eBay is probably the place to go.
It's not quite what you're looking for out of the box (but close), but Cacti will do most of those things and is pretty easy to set up. You don't even need two NICs.
My first guess would be that your ISP is blocking 222. I would try moving to another port and seeing if that works.
If that doesn't, check to make sure that you don't have any software firewalls or anything blocking the connection.
Unless the receiving server does an rDNS checkup. The majority of them do, and when a server named 'sever1.domain.com' is trying to send mail, and the reverse DNS points to something like dhcp-xx-xx-xx-xx-provider.com, it's not going to accept the mail as the PTR record doesn't match.
I would say definitely go with Linux unless you have a concrete reason not to. If you're familiar with Linux (and you say you are), then why even consider Windows without a need for something like Exchange/some other MS only product?
Set yourself up a nice Linux box, you can use Samba and...