Getting my feet wet - first all grain brew equipment questions

I've had this:Amazon.com: Coleman 28-Qt Cooler with Molded Handles, Blue: Sports & Outdoors

Laying around my garage for a while, I added a spigot to it about a year ago to use as a chilled glycol storage vessel. It worked great and held sub-0 temps for quite a long time, but I've since stopped using it.

I've been doing some reading and it looks like this would be suitable for some medium grav all-grain batches. It's already got a spigot on it, so my plan is to just use a very large nylon grain bag initially rather than fuss with a manifold. If it's worth it later I can assemble one but I don't want to go overboard on my AG first brew.

I've got a few questions however:
How big of a grain bag will I need? I was looking at just grabbing the biggest one AHS has (18x32), how much grain can I reasonably hold in this thing?

What would you guys recommend I use as a sealant? Since I drilled the whole for the spigot myself it's a bit messy, o-rings don't do the job. Until now I'd used silicone sealant and just replaced it after each use (sub-0 temps destroy that stuff fast) but I'm assuming the cheap silicone sealant from Lowes isn't going to hold up at the temps I need for mashing. Would some of that harder 2-part epoxy work? My plan is to just hit Lowes and read packages until I find something with a high temp tolerance.

I plan on doing a fairly standard batch sparge, sticking to what I read in BobbyM's primer. Any suggestions regarding basic techniques to change?

Any input is appreciated .

I recommend just doing flyguys conversion for a braid type manifold, took me 20 minutes and cost about 20 bucks. Well worth it.

chriswilkes33 said:

I recommend just doing flyguys conversion for a braid type manifold, took me 20 minutes and cost about 20 bucks. Well worth it.

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It's a rectangular cooler so if I did a manifold it would probably be of the pvc/copper tubing type.

I just don't want to complicate this anymore than I have to. If there's a real reason to spend the time and money immediately I'll do it, but I don't see why I'd lose much (if any) efficiency with the bag.. based on what I've read.