I'm only 21 so I can't speak from experience, but I say go for it. There is alot of fun to be had in the park and you would be selling yourself short to not give it a try. It's alot easier than it looks to get into, and you will be hooked and want to try bigger and better stuff in no time.
I started hitting jumps this season and I was worried about me knees because I am currently riding with a torn ACL. However, I've had no problems and I've even suffered some pretty bad looking falls, but it honestly didn't even hurt.
As for the board. I'm about 205, 5'11 and I currently ride a Ride DH 157. Its still in great condition so I'll probably continue riding it for awhile, but I eventually want to switch to a NS Evo-R 155.
The board is gonna depend on what you want to do, you don't want a super flexy board if you going to be hitting mainly jumps, but if your gonna be jibbing and doing butters, than its an absolute must.
I say get whatever size you think you'll be comfortable on, a smaller board will let you spin easier, but you will loose some stability at high speeds.
A 155 is technically too small for my weight, but I've ridden one before with no problems. Generally downsizing is acceptable for park riding.