I'd simply de-rate the 100A 240VAC sub-panel in the garage down to 70 Amps -- using the #4 conductors -- they being a sunk expenditure.
I'd back-feed these (2) breakers if necessary... but considering your scheme, a MLO (main lugs only) sub-panel with the feeders protected at the 200A panel ought to be the lowest cost. Walking over to kill that switch is no biggie.
You're never going to truely tap out even 70Amps at 240VAC single phase in a garage setting -- even allowing for an arc welder. (It's an intermittent load, anyway.)
A #6 would be plenty as a bonding conductor (green) between the sub-panel and the 200A panel.
With plenty of #4 to hand, you've always got the option of installing a Ufer ground -- see the NEC Handbook for the style. This beats a set of ground rods and is mandatory out here in California. It's cheaper whenever driving ground rods is a nightmare. (Which, BTW, they are here: stoney ground that destroys excavator bucket teeth.)
Puddling over the #4 with concrete provides protection for the Ufer once buried.
Schedule 80 is never used out this way: schedule 40 is plenty tough enough.
Wrap your PVC joints with painters tape (blue or green types) just above the joints -- which when later removed leaves a beautiful, clean glue line for all exposed work. Skip this step for buried glue-ups.
Though (slightly) more costly, my style preference is to use chase nipples into F/A where the PVC enters the bottom of exposed panels. Otherwise figure to use a plastic bushing, locknut, and a T/A for this connection.
The cheesy concentric knock-outs punched into most low end panels are tough going... if you don't want to screw them up. Take your time nibbling them out, ring by ring.
Considering the usage, I'd seriously consider buying GFCI circuit breakers. You should have more than enough breaker spaces to stuff them inside.
Trying to find a popped GFCI receptacle -- down the road -- in a crowded garage -- can be quite an exercise.
Also figure on having a few exterior use GFCI receptacles consequent to this project.
I'd always be sure to install receptacles towards the front -- left and right -- as one will always want to run an extension cord out of the garage to a parked vehicle project/ hydraulic wand for pavement cleaning/ de-icing.
Edited by Tesla (04/06/15 06:28 AM)