After par boiling the tomatoes to remove the skins, half the tomatoes and squeeze them over a strainer to remove the seeds. Make sure to put a large enough bowl under the strainer to catch the liquid. Then mash the tomatoes with the potato masher or just break them up with a spoon. I have never used a sieve for tomato sauce.
Making tomato sauce....
I use a tin of tomatoes and couple of real tomatoes and a large tablespoon of tomato puree.
In Spain we fried fresh tomatoes in olive oil and sieved them. Nothing better. If you don't want pips cut the tomato in half and spoon them out before chopping them.
What does everyone have against tomato seeds?
As for canning, I can see why you want to do it, but just check (unless of course you're an old hand at canning) carefully about how to can tomatoes as if they are not canned properly they can become very toxic.
Personally, if I had the freezer space and as many tomatoes as you I would freeze my sauce, but then again as many have already said, making sauce is so quick and easy...
Didn't know there were different brands and models for food mills. I would go for one that looks all metal and pretty sturdy like an Italian Nonna would use. They will work through your sauce in no time, and get rid of all the skins - which means you don't have to chop all your tomatoes.