how many ltrs to clean house will 250 be ok to do a days work
Not really - On a full 9 - 5 day residential day I get through virtually 400 with a varistream and fanjets. About 300L this time of year.
Some further thoughts:-
If your payload is 595kg then 250L plus you will be about 350kg - add tank, fixing kit, reel, battery (if separate), poles, bits and bobs = 100kg.
So up to now you have 450kg used up. If it's roomy enough you could just squeeze a 400L in there but perhaps only fill to 350L to be safe. Or the 250 and a couple of 25L barrels to top up with?