You're posting in exactly the right place.
The SSO-25, as you probably already know, is full of valves (tubes on your side of the pond). So you need to know that all the tubes are firmly seated before transport, keep the organ as stable as possible and then check all the tubes again after transport.
With a 50 year old organ like this, you should check on its service history, as it can be a high maintenance item. I'm thinking in particular of electrolytic capacitors, particularly in the power supply, but probably elsewhere. These are cheap enough to replace but there may be a lot of them to do. You're really looking at doing maintenance yourself as a professional engineer's fees would be astronomic, given the amount of time needed. We can give you plenty of advice on this but if you're not happy with the idea of working around very high, and potentially lethal voltages, then think carefully.
As far as the organ goes, the SSO-25 'Lincolnwood' is a well-respected Lowrey classic with a great sound, if it's working properly. It has the original Lowrey AOC and a leslie speaker built in. You don't mention what type of music you're likely to play but the SSO-25 is more of an entertainment instrument than classical. It will do very nicely with classical fare, but it's not it's forte.
As for the price, free is always good, but do bear in mind potential maintenance costs in terms of parts and your time. Keeping these oldies going is a labour of love.