MYOG air mattresses are easy to make IMO, just time consuming since not using an impulse sealer. I've made a handful of them and have learned a lot through trial and error.
For a 3-season pad I would use insulation for sure, and would also use synthetic. Easier to work with and no worries about down migration, plus not as much worry about taking a separate pad pump as with down...I personally think 70d is overkill, but everyone's camping habits are different. I've had no probs, but of course I am careful. Seems like I read somewhere that the puncture resistance of 70d over 30d is very little. Don't take that as the gospel though.
For me, the valve was always the trickiest part. I had spotty results with using the Thermarest replacement valve. It was always a chore to get it sealed consistently. I ended up using kite valves which work great, just not as clean looking. I would practice sealing your valve on a couple pillows first.
Don't forget to allow for the pad to "shrink" when you blow it up, even when using baffles. Similar to a quilt, your final measurements will be smaller than your initial measurements. Speaking of baffles, I would use the 30d material. Very little weight savings for the effort on a pad this short.