Starting Lego robotics projects low-budget with the intention of expanding later I've been digging around for cheap ways to build and control simple devices and was looking at the Mindstorms sets when I found WeDo and WeDo 2.0.  WeDo appears to be compatible with Power Functions sets, but WeDo 2.0 uses a different connector which is similar to, but not the same as, Mindstorms.

I'd like to start low-budget (suggesting WeDo as my starting point, because those controllers are much cheaper) but I want the option of expanding to more of the Mindstorms functionality in the future.

For my purposes, right now, WeDo seems adequate, but it looks like a dead end (unless the WeDo 2.0 connector is the future, in which case maybe EV3 is the dead end!). That's antithetical to the reasons I would even consider Lego in the first place.

Digging further I see rumors that WeDo 2.0 and Mindstorms may be set to converge in the future.  So even if I convince myself that i have no choice but to start with EV3, that could still turn out to be incompatible with future parts; but on the other hand it may turn out that my cheap WeDo 2.0 controller would work with future Mindstorms accessories.

Perhaps WeDo 2.0 and EV3 are compatible using just a cheap adapter, like how PF1 parts are.  I don't know.

In brief; if I want to expand my kit in the future, and I don't want to spend a lot of money today, then what should I buy?  Is there any concrete information available about compatibility or future kits that would help me make this decision?
According to the WeDo 2.0 FAQ

Is this a new plug system?
Yes, this is the new LEGO Power Functions plug that has been optimized also to meet potential future needs.
What does that mean for the existing plug systems on other Power Function and MINDSTORMS products? Will they also be changed?
Yes, eventually we will convert to the new plug system after a transition period. The exact timing of this transition has not been determined.

However, if you want to build something today, WeDo is a good choice since it is compatible with the current Power Functions products.
There is also the 3rd-party SBrick that works with current Power Functions products but uses Bluetooth Low Energy like WeDo 2.0.