You could always build a basic one and add a BD drive later on when you feel you require it.
I've never been a big fan of the somewhat 'tweaky' streaming features of BD player that's $100+ when you can customize something better. The advantage of building a cheap PC is you can choose the software you want to use and update/change as you see fit.
Whether or not Roku works better for your needs than a BD player with streaming apps is purely subjective. Test them both out and pick the one you prefer. For me at least it's the price point that keeps me away.
I've been considering getting the Roku XDS as I am currently using my laptop to stream netflix to my tv over an HDMI cable. And I was considering just using a streaming box for convenience and what not. But the XDS is about $100 and it doesn't play disks. I don't currently have a blu-ray player or disks but will likely in the future. Anyway there seems to be BDP's that have streaming apps for netflix like the XDS has... So would I be better off just getting a Blu ray player with streaming services and wi fi or is the XDS menu's and apps worth just buying the streaming box?? I would like one that is wifi ready, no additional adapters. Any advise would be greatly appreciated!