I'm a big fan of the Sunflower disc, the "old man" has a 28 footer that does a wonderful job, it doesn't leave hills or valleys from the center of the disc or the enges of the wings. This thing will cut into sod on clay ground like nothing I have ever used before, it was a huge step up from the old 254 White!!!
Sunflower | Products » Disc Harrows » 1212

 

Would guess depends on your ground as to how heavey you should look for.... Sod, rocks, clay....

Might look at what others around you are using for a good idea of what to buy....

I have been around JD's, IH's, and a Kellobuilt. All there seem to be good, but all three are built for three different conditions so not a good comparison to each other. But I like the quality of them....

 

I am not really sure you can still buy a brand new 8 foot hydraulic disk. You can buy a 3 pt. 8 foot disk from companies that specialize in making 3 pt. attatchments such as Kingcutter, Woods, Bush Hog, Frontier for John Deere and such. You may be able to find a used one easily used at farm auctions and in the local farm papers. But 10 footers are alot more common so I think a 8 foot hydraulic wheel disk is going to be hard to find used also. I think now the smallest most disk manufactuers make in a hydralic lift wheel disk is a 16 footer.

 

Checked out the website for Frontier equipment, they are a John Deere line of implements, there is a DH 15 series 8 foot transport disk made.

 

Model 155 Single Offset Disc Harrow

 

I gather by hydraulic you are looking for a drawn wheel lift disc harrow. Now comes the hard part. Do you want a plow? There is a 7' JD offset unit for sale locally here for $1500. If you want a tandem harrow disc("x" pattern disc) you will have to look. Athens Plow builds a damn good implement and is close to your area. They have a comprehensive website with a HP chart.