Which Mac to develop iPhone apps?
Solution 1:
The current low level Mac Mini with an upgrade to 4GB memory is probably the best bang for your buck. That's what I have except with 8GB; got my memory from Amazon for $130, cheaper than Apple memory. It's a very capable machine. If you use a lot of programs simultaneously, the standard 2GB is kinda cramped, but 4GB would do fine; it just seemed like a good deal for 8GB and I use more than your average number of apps simultaneously and lots of Safari tabs.
The new MacMini is much nicer than the 2009's. I know this because I had one before this one; with 2GB of course :).
You're gonna love OS X by the way. Don't be surprised if you ditch Windows in a year.
About your specific questions:
Any current Mac has an Intel CPU. Apple switched from PowerPC back in 2006 and yes you need one, but you don't want a computer that old anyway!
Just about any keyboard or mouse will work fine with OS X. If you are using some kind of fancy mouse or keyboard, then you'll probably will need some third party software to get all those custom parts working, but all the standard parts will work; left click, right click, scroll wheel, cursor control etc. There is a chance however that there is no OS X equivalent of the third party software you may be using.
Finally, Macs work with standard monitors and even regular modern TV's with DVI, VGA, HDMI inputs (virtually all of them do). The most you'll ever need is a common adapter found at Best Buy, Apple Store etc.
Solution 2:
Any current mac would work just fine. Just pick the form factor you like best at the price you are willing to pay.
I use a mac mini for iphone development and it works fine with all my standard peripherals (monitor, keyboard & mouse). I had to get an adapter for the monitor.
Solution 3:
I completely disagree with all Mac Mini suggestions.
Get a macbook. Used or new, depends on what you prefer. Or better yet - get the Air which is same price than the macbook.
Today a new mini is U$300 cheaper than both the notebooks (which goes for U$999) but you don't have to deal with keyboards, mouses and above all lack of battery. Any computer without a nobreak is a break dealer for developing, from my point of view. Adding all that plus the space used, power consumption and convenience we can easily sum up to macbook being a lot more worth it. Plus, with the air, the SSD makes it worth even a lot more.
The mini is good as a home server or any kind of dedicated machine. Never as a desktop nor to have frequent user input - even if it can be set up for such just like any messy PC.
Anyway, I'm a long time PC user, I never dropped windows or PC, but if you're going to develop to mac, please, go all in. Learn to enjoy the apple way, to hate it as well, and make that software really a Mac OS one, not a ported piece that doesn't fit in. There are many, many nuances to a true software made for mac that can make the difference for your software being successful on the apple market.
You can even use bootcamp to install windows on it if you wish, but like everyone else said, as a developer you are going to fall in love for the mac OS and the amazing hardware (which won't happen if you go Mini).