For setting up your first HTPC, a laptop does have a couple side benefits.
Embedded UPS - great for when you are still doing lots of rearranging/plugging/unplugging during planning and testing.
Embedded Console - you don't have to grab a spare monitor/keyboard/mouse when you try some new graphics driver and get no signal on the HDMI port.
All the drawbacks are pretty obvious: poor expandability, paying for features you don't need, etc, etc. If you got a bargain on the laptop, are using a network tuner, and keep your goals limited for the first iteration, then you have already mitigated most of the downsides.
My advice is to avoid scope creep. It sounds like the initial WAF could greatly determine the future of this project. Pick one or two features that are well within the specs of the machine, focus on those, and really test and polish them. Once you hit the limits of the laptop's capabilities as an htpc, you are looking at full replacement for future expansion. That can be a hard sell if the Mrs is tired of being your QA department.