While a well-broken-in Brooks Saddle is the best thing in the world for a long ride, a brand-new one would be p-a-i-n-f-u-l after the first 40 miles. If you have the time and can ride about 500 miles with the Brooks before the event, then maybe. Otherwise you might be better off with the saddle you have.

To the OP, I've seen people do the 150 sucessfuly on everything from the latest and greatest multi-megabuck, weighs-less-than-a-fart racer to a 30 year old sidewalk cruiser. Mountain bikes, hybrids, recumbents, tandems, and last year a fixie and a unicycle.

The three most important things to have a fun and successful MS 150 ride is:

1. It's not a race, it's a tour. Relax and enjoy the trip, the camraderie, and the company.
2. Conditioning yourself to spending long periods in the saddle. If you can hack 4 hours and 50 miles, in a day, you're fine. If you can hack 30 miles and 3 hours, you'll make it, but it'll be a challenge.
3. Make sure your bike is in good condition. I can't count how many people had to bail last year because of preventable mechanical problems.

The first MS150 I did I was a 45 year old, 230 lb, out of shape, former cyclist, riding an 80's era brazed and lugged Phillips that I rebuilt. I took my time, used the rest stops, and had a good time hanging out with my teammates for about 9 hours each day.

That actually kick-started my return to cycling, and last year (a 49-year old, 175 pound, daily commuter cyclist on a much "better" bike) I did each day in 5 1/2 hours, including rest stops. I felt good about the improvement in my physical condition, but after I finished I realized that I had more fun the first time.

This year, I'm going to try to balance pace and pleasantries, speed and scenery, and see if I can have the best of all worlds.