Is it fine to buy a lot which has engineered depth fill of 1

Are you missing a decimal point....?

The only way it will impact the building is through poor design of the footings and slab. Providing that doesn't happen it shouldn't have any bearing on any insurance costs on policies. I could be wrong but I've never heard of it being mentioned before.

Generally, the only thing which affects resale value would be if the structural design was lacking and the building showed signs of distress-which it shouldn't if done properly. Make sure you get a decent geotech-the minimum requirement to fulfil the building regs will be of no use, it will (or should) tell you its a P site and a 'deep' footing system is required and look to minimise those costs upfront.

Be aware that compacted clay fill has an increased potential for shrink-swell movements compared to the same soil in a natural condition.

Heaps of lots these days are made up of engineered fill. In all likelihood the site classification would come down to a P increase your site costs in the range around 10K. The builder's engineered plans should take care of building on such site but I have seen even the big name builders going with regular waffle pod slabs without footings on such lots.

insider on sites like that is there is any other real alternative to a thick (250-300 mm) reinforced fully floating slab?

You either go deep or you go with more reinforcement/wider footings (with a slab being just one big footing).