Hello Ed,
I follow this with great interest because I am currently designing a system/battery monitor / solar charge controller w/MPPT / dump load controller using an AVR (ATmega48). After I get that done, I'll go for the wind generator controller using the same chip. I've studied the AVR pretty thoroughly now (including the ATtiny45), and have quite a bit of experience programming (though new to AVR), so if you have any questions, I'll be glad to try to help.
I really like the tiny45, it has PLL to run the PWM at up to 64MHz (8-bit resolution at 250 kHz), differential input and 20X gain on the ADC (very useful for the current input), and it's cheap, although the mega48 is pretty much just as cheap and has a lot more pins (28) and a 16-bit timer.
So, what speed are you planning on running the PWM ? Are you using C or AVR assembly ?
For your relay, I would just use a MOSFET; it has much lower drive requirements, and since you are not turning it on and off fast, the gate capacitance is of no importance. For the design that is currently in my head and partly in computer files, I originally thought it would have a relay to enable the controller, but later decided that the buck MOSFET would do just fine. Even if I were just doing a boost controller, I might just skip it.
Flux (and SamoaPower) is right about the turn-on and turn-off of the MOSFET. At the very least, follows Flux' recommendation about the parallel capacitor, and if you are ambitious, you might want to consider soft-switching (zero-voltage turn-on and zero current turn-off). This takes another MOSFET and another (quite small) coil, but it greatly reduces the stress on your primary MOSFET and increases the efficiency of the boost converter a bit.
For those that commented about feedback for the PWM, things are different here from hardware PWM chips. By inputting the battery voltage and wind genny RPM (you don't really need the wind generator voltage; you'll know that close enough from the RPM), a microcontroller can easily compute what the duty cycle needs to be and have the other 98% of it's processing time to sleep.
By the way, when I first starting investigating this, I thought it would be great to have the microcontroller update the duty cycle really fast, and feedback on the output current to get maximum power, but this is unnecessary. The inertia of the rotor also causes problems with MPPT since an increase in the duty cycle (buck or boost) will instantaneously give more power, but it is just bleeding off some energy from the rotor inertia. Now I'm thinking about 2 to 5 Hz or so, without current feedback. Since the TSR plateau is pretty broad (about +/-10% from optimum RPM), it does not have to be tracked that closely.
Keep us posted and best regards,
Dave