If you have a MacBook Pro with 10.5, that's usually a machine that's getting long in the tooth and thus running Windows as a virtual machine (one OS inside another) will likely be slow.
Also spending all that money for Windows (VirtualBox is free) and Office seems like almost a waste of effort if the machine is going to die suddenly.
MacBook Pro's last about 4-5 years.
To do what you want your going to have to upgrade OS X to 10.6, then either buy VMFusion or Parallels which they are now catering to 10.7 and 10.8 users, so it's slow on 10.6, or use the free VirtualBox which will run well on 10.6. Then buy a copy of Windows 7 as earlier Windows versions are not being sold, then Office 11 or whatever.
You can also install OfficeMac into 10.6, if they still sell a copy.
You see your outdated really, and anything that can work is soon to be ignored etc.
What I would consider doing instead, since your not a OS upgrade type of person (we are on 10.8 now which won't run on your machine or be extremely slow) is to buy yourself a decent Windows 7 laptop and install Office into that. Some have the demo already inside and all you have to do is activate it.
This way you get your Windows experience and Office experience, don't have to fiddle with complicated virtual machine software, or Apple's problematic Bootcamp etc as Apple really is leaving 10.5 and 10.6 users behind, and by next summer will be leaving 10.7 users behind too.
Windows 7 laptop will be the cheapest and least stressful option for you, you can continue to use the Mac until it drops dead of course.
I say this because your on 10.5, your like most computer users, just a computer user and not a geek.
You obviously have some reason you need Office, likely for a job or school and since 99% use Windows and Office, then that's what you should be using.
If your only needing Office or Office like program for something temporary, then I suggest you install the free LibreOffice into OS X, however I don't know if it will run in 10.5, you might have to upgrade to 10.6 via the 10.6.3 white retail disk, however Apple doesn't sell it anymore, but perhaps Amazon or elsewhere does.
Sorry, you have been out of the upgrade loop for quite some time, so a new machine is in order as that one is about to die soon, so I wouldn't be spending too much on it really.
If you have a fat wallet you can buy a new Mac with 10.8, and have Windows 7 installed in BootCamp (direct boot) and also into VMFusion or Parallels (copies the Bootcamp Windows so you can use both) and Office for you. It will come all set up, enter your password and use the machine.
I think OtherWorld Computing will sell you a pre-configured Mac all set up like that for you, Apple won't do it though.
You can also pay a local PC/Mac specialist to do the same.
Don't forget setting up a backup system for both Windows and OS X, add anti-malware software in Windows, you'll need that.
IMO a new Windows 7 laptop would be the cheaper, less stressful option, Learning Bootcamp and virtual machine software can be a pain if your not computer savvy, al;so Apple has a nasty habit of upgrading the operating system constantly, now on a annual basis. You don't upgrade and all your third party software, then your denied security updates in short order. Windows 7 laptop will last a decade on the same version, only needing simple updates, not upgrades.
If you want your Windows machine to last even longer, like a decade, then get a desktop tower instead of a laptop.
Mac's are for those who can handle a lot of change, as Apple is about always inflicting change. It's costly change too.
You still need to upgrade OS X to 10.6.3 via the white disks, call Apple Sales on the phone perhaps they can still sell you a copy. Or try Amazon.com. this is the 10.6.3 white retail disk, it's the full 10.6.3 or it can upgrade 10.5 to 10.6.3, you will need to use Software update under the Apple menu to get to 10.6.8 before you install Parallels or Windows 7.
With Windows 7, use the 32 bit version, should give you a bit more performance on that older machine.
Also your going to need at least 4GB of RAM for your computer and install it yourself.
See Crucial.com for that and use their online checker, make sure you know what machine model and year you have.
RAM install videos and tools are here
Don't forget making a backup of your files.
If your machine slows down after upgrading to 10.6, the you can clone and reverse clone
or fresh install 10.6 again from the disk