Run the engine for thirty seconds without the belts. See if the noise reoccurrs. If not then it's probably the water pump, could be the impeller coming loose.
I can recommend the Haynes manuals as a start point if you prefer to read. The actual factory manuals can also be found in line and from booksellers. Britannia restorations, Mike, from Canada, on YouTube, is excellent.
No series engine has a timing belt. Sounds like it could be a fan pulley coming loose and catching the cowl, or an alternator bearing. Check the tension on the fan belt.
Petrol? Standard starter?
Solenoid on the battery tray. White (& red?) wire from ign switch to solenoid. Battery - fat red to solenoid and another fat red to starter.
Some of their kit is excellent. The CVs that the coil sprung lot use are the best in the market. There's a few other examples out there. Most of their stuff is meh. And there's far too much crap as well.
Infuriating, to be honest as a tiny bit more effort from them and they'd be a great supplier.
I've it set a bit too low, as I'm planning on increasing the boost on the turbo quite a bit in the next round of upgrades. It came on last summer in traffic. I was towing a trailer with a Bridgeport onboard at the time. The fan only ran for a minute or two.
I bought it on eBay.
Head off, I remove as much as I can, makes lifting the thing easier. New gaskets all round. Composite gasket.
Up to you on turners heads, I fitted the head from a 2.5 on mine.
Assume exhaust studs are poor, if inexpensive, get anyway.
Without knowing the regulations impossible to answer. As above, Britpart are, unbelievable as it sounds, a supplier of genuine parts, so it's a service replacement, not an after market part.
As above, fit with no washers. And don't tighten. Then add washers when your are doing the alignment. You can cut a chunk out of the washers, so that they are U shaped spacers instead. Saves you taking the bolt out ....