It's changed from Challenge to IM this year so until they put the details out it's hard to answer. If however it's the same or very similar to the Challenge course, the bike course is probably medium difficulty (but you're talking to someone who doesn't mind hills).
It was 2 laps with about 850m of climbing per lap, with most of the climbing in the first 1/2 - 2/3 of the lap, but I have to say I was surprised there was that much elevation as I didn't feel it was that hard (there's nothing too steep or long to deal with). The last section is relatively flat and can be fast if the wind isn't blowing in your face.
I haven't done outlaw but from what I understand it's a relatively flat course - but, bear in mind that a flat course can be as hard if not harder than a hilly course, because a flat course you have to push the whole time & don't really change positions; at least if there are some hills you can change position, use different muscles and generally get a rest on the way back down!
No idea about immortal sorry but there is a recent thread on it you could have a look at.
If you don't like sea swimming then personally I wouldn't go for Weymouth - I'm not a fast swimmer but can keep going for a long time, but that was my first sea swim (well, "competitive" swim - I grew up in Australia so have swum for fun in the sea plenty of times!) and I did not enjoy it a single bit. So personally I would say unless you have your heart set on it for a particular reason, if there are a few you are tossing up then especially for your first I'd go for what will suit you best.