First off, I would like to know how you managed to get the wife to buy you a XJ for Xmas?? Mine would get rid of it in a heartbeat if she had the opportunity... I'm all ears. Valentine's day's coming and a second XJ would be nice.
$400 is a fair price for RMS replacement. Let's be honest, if you're not entirely confident in your mechanical skills, this is going to be somewhat of a challenge. To begin with, it's MESSY; you're going to be covered in oil. I would actually do an oil change shortly before... just because I'd rather take a bath in clean oil. I would agree that the main difficulty is removing the oil pan due to the lack of clearance between the engine and the front axle (of course, if you have a lift, piece of cake). Just be prepared to be throwing a few cuss words, regardless of whether you're prone to it.
Follow Cruiser's advice and make sure it is indeed the RMS. Absolutely use a brass punch for the upper piece. And you might want to start with the valve cover gasket to warm up your skills; compared to the RMS, it's a joke.