Peter, welcome to the club! I don't own a Series bass, so I can't speak from experience; but I gather from the posts I've read here in this group that it's rather impractical to own a Series bass without the power supply if you play it a lot, as it's generally said that they eat batteries quickly.
Also, you may want to take some time and read a lot of the older posts about Series basses and educate yourself about them. The Search feature of this board works well for that purpose and there's a lot of great information to be found here.
As far as the price, it's within the range of what I've seen older Series I basses go for on Ebay. So if it's a good example of a Series I from that period, the price is not unreasonable. Again, most used Series basses will have the power supply; so you may wish to consider the cost of acquiring another power supply when considering the asking price, keeping in mind that used power supplies are not common.