If it is 3 phase it will be a lot easier to start, but as the rest of your loads are likely to be single phase you would have to spread them fairly evenly round the 3 phases for best results.

I would be inclined to over size the alternator, perhaps 10kVA, but you can probably manage with about 10hp on the engine.

10hp and a 10kVA alternator will manage a reasonable size welder. You could consider a separate welder generator as it is not likely to be used often.

It depends a great deal on the type of alternator. A good stiff alternator with voltage regulator of 4kVA might start a 2hp motor off load when a poor 8kVA wouldn't

Motor starting and welding present a reactive load which will tax the alternator but are not so demanding on the engine.

Heating loads are the ones that will mop up the engine power.