I have built a lot of stationary engine trolleys,to carry engines up to 15 cwt, a trolley for 300 kilos does not have to be substantial ,a lot of trailers for towing behind cars only have 50mm sq box section with 3 mm wall thickness for the axle and at 4 ft long will carry half a ton. As for a turn table ,it only requires a pair of steel plates 4 to 6 inches dia with some grease for lubrication and a couple of holes to take a bolt so that the steering can be locked to steer straight.The big problem with machine tools are that they are always top heavy so it is advisable to have long axles for stability ,for travelling over rough slabs or uneven concrete it advisable to have wheels larger than castors and have soft rubber tyres so that there is some "give" when a small ledge or an uneven surface is encountered.To move loads uphill try using a trailer winch,mount the winch on the trolley and attach the hook on the cable to a convenient anchor, i.e. a tree, shed floor or concrete path with a Rawl bolt ,thirty years ago I bought a 30 cwt Blackstone which was mounted on plastic wheels with rubber tyres ,wheels were 14 ins and 9 ins dia,it was amazing how the wheels took the load,later ,when the engine was mounted on a decent base I kept those wheels and made a steerable trolley like those in garden centres, and it gets used all the time,very often to carry an engine during restoration and it still carries half a ton easily,so don't over engineer a trolley. I have a pallet truck,its great on level floors,but useless on uneven ground,trolleys with castors are dangerous, the wheel base and track varies too much when being moved and can tip over very easily.