Many of the older 4 Stars had a "milled" finish of the exterior panels. They were a duller finish than the trailers being produced today. An acid wash will clean them, but not make them as shiny as you might expect. The exterior skin can be mechanically polished to higher reflectivity, but it is labour intensive.

Because of the 4 Star's sheet flooring, an aged trailer will have divots in the flooring. Where the horses stand, the metal will stretch and result in a small pocket allowing fluids to collect and stand. When the mats are installed, the trapped fluids cannot evaporate or migrate, and these areas become a breeding ground for corrosion.

The best fix is to pull the mats and rinse the flooring. When standing water is realised in certain areas, drill a 1/4" drain hole in each puddle, allowing the water to drain. Depending upon the severity of the depressions, you may need four holes per stall, or a little as one.

I would seriously consider an acid washing of the flooring, to ensure any corrosion issues are addressed. Following up with a bed liner material would greatly reduce any further maintenance issues.

Check the spring latches on the stall dividers for wear. 4 Star used a spring loaded pin that fits into an orifice let into the side wall panel. A stainless patch is fastened by blind rivets to reduce wear on the aluminum structure. Both the divider and the side structure are subject to wear, from the bolt like latch on the divider, to the hole for the pin. As they elongate, the pin can bend, the divider, snubbed by a piece of rubber can begin to rattle, the bolt like latch handle can rip out of the divider. These items are difficult to repair, by replacing sheet metal and the mechanism.

Check all the window and door latches for integrity and operation. 4 Star uses heavy duty components and usually there will be no need for any maintenance, other than an occasional squirt of lubrication. The various hold backs are rived to the body instead of being screwed into place. This makes their replacements a little more difficult.

On that year trailer, unless the previous owner has done so, you will need to replace the brake and wheel bearing assemblies. This will cost several hundred dollars, but then they will not need to be again replaced, for probably as long as you own the trailer.

A new replacement trailer will cost in the range of ~$24K, depending upon what options you specify. $8900 for a twenty year old trailer is a bit high, especially if it needs an acid wash, tires, brakes, bearings, a new plug and maybe some lights. A spare tire and wheel can cost a couple hundred dollars, hubs caps are inexpensive, SIMMs more so. You may end up putting $500 - $1500 into the trailer after purchase, to upgrade and ensure reliability. Once that is done, you will not need or anticipate any major expense for many years.

I would offer $7500 and settle on about $8000 as a maximum. If the mechanicals are done, the tires are good and a spare can be furnished, maybe another $500 max. for a total of $8500.