The guide is a lot softer material than the valve so I would be very surprised if the valve showed any measurabe amount of wear. I would feel comfortable ordering standard size guides...all of them. Don't just pick and choose, replace them all. I would say #5 is definately burning a lot of oil. It is very probable that the oil in the cylinder caused detonation, which in turn blew out the head gasket. The 103 just won't take much detonation before blowing the head gasket. The steam slots between the cylinders give the pressure an out and when detonating it takes the out. With the 3.6 having such a small area in the fire ring it is even more prone to blow the gasket. The upside being that a head gasket is cheaper than buying pistons and doing a bottom end rebuild.

The valve seats should be fine unless the head has had a bunch of rebuilds. After the new guides are installed you just need to cut the seats enough to get new surfaces for the valves to seat on. A nice multi-angle grind on the seats may pick up a bit of low lift flow while you're at it but not a necessity by any means.

When torque-ing the head I like to do an initial pass about 1/2 of the first stage. Starting with 20ft/lbs gets the head snug on the block without as much flex as cranking the center bots to 45lb/ft right off the bat. I don't think head bolt torque caused the failure, I just don't like to put 45ft/lbs on the first few bolts with such a long aluminum head.