some good examples amongst the ebay listings, although I would avoid the third one, as condition of the chapter ring is suspect.
I have owned a Spaceview in the past, and lusted after one until it arrived, at which point I was underwhelmed. Purely personal taste of course. I settled on an Astronaut which I absolutely love, it has the same 214 calibre movement but the bezel gives the watch more overall presence and the 24 hour hand is a nice addition.
The issue of conversions is an interesting one, as a lot of Spaceviews were converted by jewellers in the 60's. My understanding of the story is that the Bulova display used a watch with the dial removed to show the new tuning fork technology, and that customers soon started asking if they could buy the watch without a dial. Bulova would send out kits to convert plain 214 watches in to Spaceviews.
price wise, I would always factor a service in to the equation, regardless of what the listing says. We are fortunate to have the Croydon Service Centre, run by a watchmaker that used to work for Bulova, he has much experience of the Bulova tuning fork models. I would recommend that you send your watch to them as soon as you get it for a service. On this basis, i think you will need £500 to buy and service a good example.
It is worth mentioning that the movement was designed to run on a mercury battery, and sometimes it is necessary to fit an Accucell battery to get he watch to run correctly. the Accucell is just a 1.55v battery with a capacitor to reduce the voltage. This can all be organised as part of a service. also bear in mind that you can't expect much more than 12 months from a battery, fortunately, the battery change can easily be done at home as their is a separate hatch, like on a Swatch watch.