will I be able to run B3D at all
The short answer is yes.
When people say later DX versions are "not backward compatible", they mean the DX7 API no longer exists as part of the greater whole. Instead, DX7 is emulated on top of whatever you've got, which is a little slower. If you want better performance, you should still be able to install DX9 alongside DX11, which will give you "real" DirectX7 calls; how much effect this actually has seems to vary.
There are a few known bugs that may be caused by a combination of Windows 7 and newer graphics systems, but they don't affect everybody (and all of them have workarounds at the very least).
I wouldn't worry about it in the long run; by the time Microsoft drops support fully the emulation/virtualisation will definitely be fast enough to give better performance than what B3D was designed for anyway (in fact it probably already does).
On the other hand, don't let yourself get held back by your oldest technology; if it were a choice between B3D and a new computer (which it isn't, luckily), I would go for the new computer and use something else. It's probably not advisable to start many large commercial projects in B3D at this point in the lifecycle (at least until the emulation thing settles down and we have a full DirectX7 again).
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