"... does not cost so much, and weight and take so much space."
That is such an absurd statement.
Cost = $300 PLUS the cost of a computer to put it in
Weight = 1 pound + the 50 pounds of computer, monitor, printer, cables ...
Space = 1/216 of a cubic foot + the 3 cubic feet of computer, monitor, printer, cables, ....
It's cheap, light and space-saving ONLY because you already have the computer to put it into. However, there sure as heck won't be anything portable about it, will there?
40MS/s is an extremely slow sampling rate and won't come close to supporting the 100MHz bandwidth. Even Tektronix's low-end TDS220 with its 60MHz bandwidth has a sampling rate of 2GS/s! And it's cost NEW is only about three times the cost of this little computer board you're considering. It has built-in waveform storage, direct-to-printer output as an option, RS-232 or GPIB interface options, and all sorts of built-in waveform calculations.
I'm not saying you should go out and buy one of these little guys new. No. I'm saying that a PC-based scope is a far cry from a decent oscilloscope. If you're going to spend that kind of money, invest it in something worth the cost. A PC-based scope has a limited lifetime -- look where PC interface busses have gone since 1982, let alone the external interface capabilities.
ebay will provide you with a lot of low-cost, high-end scope choices in the used market. And don't be like a lot of folks and pooh-pooh analog scopes. Most medium to high-end analog scopes will run performance circles around most digital scopes in the same price range.
Dean