you do not need holding tanks for BSS as to the cost without going through the boat
nobody can guess the cost,has she had a BSC in the past that has expired?
see link for full BSS requirements
If not had one before then will probrably need the usual bit doing ie fuel lines to iso7840, batery terminals crimped type , lots of labels, gas test point and posibly gas storage locker if its one with door into aft well, if you are now buying then try to get a surveyor who can advise at the time of the survey.
If its just a renewal then a test fee should be all thats required, about £150 ish depending on area