Purchase Mac OS X 10.6 on DVD, run the Mac OS X 10.6.8 combo updater, and then use the Mac App Store to purchase Lion. Nobody here knows or is allowed to say what the system requirements for Mountain Lion will be at this time.
If you're using Office 2004 or any other PowerPC-only software, you won't be able to run it in Lion. Otherwise, everything will remain as is unless a problem occurs.
Your Mac can upgrade to Lion. The requirements for Mountain Lion are unknown at this point. First you need Snow Leopard (10.6). The disk ships with 10.6.3, you will need to update it to 10.6.8 by simply going to the Apple menu (upper left) and clicking Software Update. Once you have the latest version of Snow Leopard, you will have the Mac App Store. Lion can be downloaded from the Mac App Store.
Updating will not cause anything to be deleted, but some apps may not be compatible. If you have Office 2008 - Office 2011 you will be fine.
Your MBP will run Lion.
For Mountain Lion the Advanced GPU chipset is required. So you will have to check that out.
Will help you determine if your MBP is capable of running Mountain Lion.
Apple has not yet announced Mountain Lion System requirements. Until they do on http://www.apple.com/macosx/ or http://www.apple.com/pr/ anything any other website says is pretty much rumor, and subject to change. http://www.apple.com/feedback/ is a good place to post requests for Mountain Lion compatibility.
https://discussions.apple.com/docs/DOC-2465 will give you the low down of whether or not your are ready for Lion. You can upgrade to Lion without installing 10.6.6, 10.6.7, or 10.6.8 first. It just requires erasing your hard drive, and using a USB Flash drive version, or installing it on a hard drive with no operating system.