Just connect your old Sky box to the dish and it will act as a Freesat receiver. No card is needed. The only minor problem might be 'regionalisation' of the box as it is likely to default to London. With BBC regions you can find them individually in the EPG slots number from (about) 960 onwards. ITV regions need to be added as 'Other Channels'.
If you want HD then you will need to buy an old Sky HD box and connect it exactly as above. However the PVR functions in Sky+ and Sky+ HD only work if you have a valid Sky subscription so you won't be able to record/playback.
Many Sky customers have simply canceled their Sky subscriptions and continued to use their installation as Freesat.