1. PVC or flex are both good, and I have used both. I am currently in the middle of a rebuild also, and I will be using rigid sch 40 PVC 4" as the piping leading from my bottom drain. 2. I have a pump hidden inside the skimmer and another at the last phase of my chain of filter tubs, which will draw from the BD. As Joe 09 mention, an external pump is an excellent choice. They are expensive, but I'm told they are more energy efficient. 3. I know of no reason why burying the biofilter would affect the efficiency of the bacteria. Just make sure you have a suitable method of cleaning, such as being able to lift the bio bags to rinse in a separate tub of pond water. Most people have them above ground, though, so they can simply aggitate the media and then open a valve to drain. 4 & 5 ?? 6. I just use waterfall foam purchased at Lowes or Home Depot. I've tried cheaper varieties, but for some reason they didn't set up properly.

Unless you're Mini-Me, the space is used for storage only. If it were mine, a surface pump or shallow pit would be all I would invest in. Plus taking the precaution to store anything in the crawl space on top of homemade pallets; just screw together some OSB and 2x2s.

Maybe think of your porch floor a little more like a roof ... I, too, have an old house with incorporated porches (front and back) that sit over top of a full basement foundation (brick with original stucco.) We restored the back porch as a "sunroom" (it was closed in some time in the 30's) and I noted that roofing tar paper was used between the sub floor (plank) and the original T&G porch floor. There are shreds of roofing tar paper hanging on the edges of the bottom of the front porch floor where I can find it, but the old wooden floor was replaced with plywood and then the porch floor was tiled by POs (a bad DIY with 1 inch grout lines that leak like crazy, making my repair job that much more difficult.) The pitch on both porches is good (away from the house) There are low shingled walls all around the porches so water would be trapped without some sort of system to remove it. The front porch uses a pitched trough with 2 exit tubes at the foot of the short wall but I found no evidence of this on the back porch. These 2 devices (the roof paper and the trough) surely would have worked well if they were maintained, alas, the metal trough is rotted thru and leaves/critters were getting into the basement till I discovered the holes and band-aid-patched them till I can address repair of the trough properly. (interestingly, the front porch foundation was hidden behind a finished wall in the basement and was only found when we had a termite inspection done.)

Here's how I would approach your situation: a. Read the great article on conditioning crawl spaces as with removal of the exterior vents, you inadvertently started down this process, which imho, is a good process. Here is the great article: Martin Holiday: http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space b. Lay down the minimum 6 mm water vapor over the crawl floor. I did this myself and it wasn't very hard: Lowes and HD sell it. Just overlap the edges and use the water proof tape, ideally bringing it up the concrete walls 10 inches or so and securing it. If you can't do the later, just lay it all the way to floor-wall junction with no exposed dirt, until you can do it as it (water membrane over all exposed dirt) is like the main pivotal step in controlling humidity of the crawl space. It's all reviewed in above article. c. Get your HVAC guy back to cut a small 3" or so supply vent circular hole in your metal duct. This plus him connecting a return duct to a floor above the crawl should provide enough air exchange. The numbers of cfm are above. If he truly won't (and he should read Mr. Holloway's article and answer why not) then perhaps you need a new HVAC man as this is established crawl space conditioning components. Remember that "perfect is the enemy of good enough". It all sounds complicated, but you've already done the hard part: removing the vents, insulating the rims. Now just lay the waterproofing vinyl membrane and try to get some air active exchange through there and you will be way beyond what most crawl space owners have done to ameliorate crawl spaces. Then sit back and take pleasure in your Home Owner accomplishment! Regards, SW.