Welcome MIdeerhunter!
First things first, where did you find a Deluxe for $175.00? That is a steal at that price. Please PM me if you don't want to post the business. Do they have any 20ga guns for that price?
OK, now to your questions. These guns are very durable, reliable and VERY accurate. I have a 20ga and absolutely love it. I wouldn't trade mine for any other slug gun on the market. Before I forget, yes the Ultra 12ga is a 10ga blank bored to 12ga and is mounted on the 10ga frame.
As far as slugs, in my 20ga, there are several that shoot very well, but none of them are cheap. The Remington Ultra Core-Lokt, Winchester Partition Gold, Winchester Platinum Tips, Barnes/Federal (all copper), Remington Copper Solids and Lightfields all shoot very well in my 20ga. These all range between $7.99 and $11.99 a box of 5.
Depending on how far you intend to be shooting, if under 100yds, the Rottwiel Brenneke and other foster slugs have shot well. The only downside is they will lead up your barrel and that requires a good lead solvent, brass bore brush and a lot of elbow grease to remove. The 2-3/4" 1oz Brenneke slugs are a harder lead (made for rifled and smooth bore guns) and will not lead up your bore as quickly, they also cut a hole due to the sharp rifled fins and leave a blood trail a blind man can follow. These slugs can be had for $1.99 to $3.99 a box of 5.
I shot a real nice 8 point last fall with the 20ga Remington Ultra Core-Lokt 260 grain and dropped him in his tracks. The bullet went through both shoulders and was found in a perfect mushroom under the hide on the far side. I couldn't ask for much more than that.
The 12ga bullets are all heavier, in the 385gr to 438gr (1 oz) range. I have shot the 12ga Remington Ultra Core-Lokt 385gr out of my 12ga autoloader, one ragged hole at 110yds but very brutal on the recoil end. With your 12ga USHD, I don't think that will be a problem since it weighs around 9lbs plus the weight of you scope.
Hope that helps a little and good luck with your new USHD!