You should do fine at 300 yds with your system. The .204 will shoot farther, though. I've been smacking steel plates the size of a coyote at 540yds. But, I'm shooting a varmint style bolt rifle.
Hmmm...Millett. Not saying Millett is a bad scope. I have one and it's heavy duty and good optics. The turrets don't have as positive a click as I'd like but it does track well. However, the darn thing jumped zero on me and I wasted a lot of time and ammo using it while testing. Mind you, all mechanical devices can fail and I might just have gotten one of those. All the same, it left a bad taste in my mouth.
So, don't put too much weight into my evaluation.
If you have the $ to put into a good scope, I'd recommend checking out the optics forum at sniper's hide. There is a lot of information to be gleaned from what those guys have used. Don't go hog wild and spend kilo bucks on a scope like some of them do. For what you want, it isn't necessary. Find a scope that will meet your needs, is reliable and fits your budget.
You'll probably need high rings for a 50mm lens. I'd recommend Burris Signature Zee rings. They have a poly insert that will prevent ring marks on the scope and negate the necessity of lapping the rings for alignment. Plus they come with an extra set of eccentric inserts that will give you a cant to help increase your scope's elevation range.
I favor the milrad/mildot reticle as you chose. But, if you're going to use it for ranging (which is what it was designed for), I wouldn't go with MOA/Mil. Pick a scope that has the turrets that match the reticle. Otherwise, you will have to do extra calculations in converting Mil to MOA. Also, be aware that with the SFP scopes, the reticle is accurate only at one magnification, usually at max (or even divisions thereof with additional calculations).
With an FFP scope, the reticle is accurate at all magnifications.
Some people don't like the FFP because it looks like the reticle changes size depending on mag setting. It actually maintains the same target to reticle ratio. Not so with SFP. But, the SFP can maintain an apparent small size at high mag. It's user's choice.
Here are some examples of what an FFP reticle looks like at different magnifications:
The actual view through the scope is much clearer than the pics show. It's just hard to get a good camera focus shooting through the scope.
Here's another link to read. It discusses the main topics you should consider before you pick a scope.
http://www.snipershide.com/forum/ubbthr ... ost1938091
Hope that helps and didn't confuse you more.
Just thought of something. Give Scott at call at Liberty Optics. He's a standup guy and can give you some advice.