Welcome aboard!
From the pics, it appears to be an 81-84 model. Â How many miles are on it? Â Original speedometer, etc. and all in working order? Â These bikes can have a tendency to have speedometer/odometer problems, but the bikes themselves can last quite alot of miles. Â Having an accurate estimate of the mileage can help determine the remaining service life or anticipated big mechanical repairs.
Does the seller have records of maintenance on the bike? Â If so, are there any records of valve/head work done on the bike? Â These bikes can often handle 100K miles with normal wear and tear, but a weak link in this vintage is the valve seats, which can cause valve problems as early as 40K miles or as late as 90K miles, depending on the usage, environment, fuel quality, and phase of the moon. Â In general though, they are pretty reliable and easy to work on, as long as you are aware of a few of their quirks!
A standard seat and rear cowling and underseat storage bin can often be found used on ebay, and from breakers/resellers like repsycle.com  Also you can use our Trading section - I think that there may be some members who may have some bits you could use for sale, or if not, that solo seat on your bike is actually a not so easy to score item, and some people may actually trade you a standard seat and rear cowling straight up for it if you don't like it.   Rob Valdez (member on this forum) is selling some stuff from his bike that could provide the rack/storage you're looking for.
Also, you can check out
www.ibmwr.org/market for airhead parts for sale/wanted, and if you join the BMWMOA.org you can access their forums and classified sections too... then there is also the BMWRA.org group, the Airhead Beemers Club...
Enjoy!