Don't buy a $800 cream, that's insane! Urea is a common moisturizer ingredient, and there are many reputable brands of urea on Amazon:
You likely won't see much difference between 30% and 40%, in my opinion.
There are some combination creams. Topix sells a product called Glycolix that's 15% urea and 15% glycolic acid. Studies have shown that urea works even better when combined with salicylic acid, and glycolic acid should be similar. It's also sold under the brand name Replenix, for some reason. It's a bit expensive, and I've not tried it, but it should be good.
See our descaling guide for more.
Good question. Personally, I like PurSources 20%. The cream is light and not greasy, and it absorbs quickly. It has a menthol scent, but it's not terrible; apparently the 40% is unscented, if you believe the comments on Amazon. Not too expensive, either. It also contains tea tree oil, which is known to have an anti-inflammatory effect; however, I don't know if the concentration is medically significant.
Eucerin is a great brand, but the only one with 30% urea, I believe, is Eucerin Rougness Relief Spot Treatment, which comes in a fairly 2.5oz small tube. The bigger lotion doesn't say what concentration, but the spot treatment says it's 3 times the urea concentration of the lotion, so most likely 10%.
Udderly Smooth has a 20% urea cream. I've not tried that one, but it's a popular American brand.
You can also get shampoos with 5% urea (Eucerin and SebaMed, for example), though the concentration isn't very strong, and you have to wait at least 5-10 minutes before rinsing to get any effect. But this could be good choice as general-purpose shampoo, in addition to the overnight descaling.
(For the record, I wrote the descaling guide.)