I have a B2620 which is a 26 HP tractor. I find my 60 inch tiller pushes it to the limit. I have to go very slow in heavy untilled clay but can do it. I would think on your 18 HP tractor the 48 inch would be as big as you would want to go.
We sell a new Tarter 4 ft geardrive for $1595 + shipping. Ken Sweet
Find your best price and take it into TSC and they will beat the price, or at least that is what they do at the TSC I go to. I found a Tarter 5' tiller at Lowes (yes Lowes) for $1505, and TSC sold me one of there's (made by Behlem) priced at $1700 for $1500 just to beat out Lowes, which was good as Lowes was going to have to order it in (when it became available at their store) and TSC had it in stock.
We sell the Phoenix T4-48 for $1475 including commercial business delivery within 1000 miles of Wilson, NC. Ken Sweet
The King Kutter XB gear driven tillers are specifically designed for small compact and subcompact tractors like the B and BX kubota. We can ship that tiller for free to a business in Afton or to the freight terminal in Des Moines. Here's a short video about it:
I would love to buy this unit from you, but i am 200 miles to far out for the free shipping
We sell a new Tarter 4 ft geardrive for $1595 + shipping. Ken Sweet