If there has been signs of slipping and a new radiator installed, plus the fluid appears brown (on the dipstick, I'm assuming?)...then I would pass on it. For starters, these transmissions don't have filters, but screens, which usually don't get serviced unless it's being overhauled. If there is enough debris in the fluid to restrict the flow, then the trans needs to be rebuilt, not serviced. The factory fluid looks brown when drained, but will look a very pale, pink color on the dipstick when checked; some people have even described it as clear or amber. When aftermarket fluid is used for servicing, it will usually look the typical red color on the dipstick. It's possible that the radiator cooler did fail in the past and even if they flushed it out, it is very hard to get out from between the TCM and valve body without removing the assembly and taking the TCM off the valve body to clean under it. If left, it can cause shifting issues which may lead to slipping or shifting issues. There are a lot of R51 Pathys on the market, so I would look around at other options. At the very least, get a CARFAX report on it and have an independent shop or dealer check the the vehicle over and let them know of your concerns. Good luck!