Seat:

cheapest option: get a used sheep skin baby cart liner off ebay (~5 EUR). Worked for me all the way to Nepal. Alternatively take the seat to any upholsterer to redo the cushion. Gel inserts are great for continental climates, but can become a bit stiff below freezing and a little toasty in the desert ...

Gauges:

- Overheating is not a problem on the KTM Mil. If you intend to use it only for stop and go in very hot cities, fit a second fan and put it on a switch.
- Empty gas: errr... hardly any bike has a fuel gauge, let alone an accurate one, especially in this price class and on an enduro. But you have the reserve and the trip counter which you zero when you refuel, so you know exactly how much mileage you have left. Easy and the same as any other enduro.
- Battery/alternator: even rarer to find on a production bike than the fuel gauge and of little practical use as the battery will be dead by then ... take a cheapo multimeter instead if you must

If you want gauges for the above, and want it OEM, then swap the bike for a GS or a big adventure :P Alternatively, put on a rallye computer with the required sensors, but with the whole shebang we're talking 300-500 EUR. I am just converting my tach to a rallye computer (SIxO Homepage), but not because it's really neccessary, but because I'm back home and fiddling with the bike eases the desire to set off again. (plus the analogue tach cable broke again, and I'm fed up with it ...). The Touratech IMO (as found on the KTM Adventures) are another, more expensive, option (if you want comparable features). For just a digital tach that fits the original housing, try the MAE-Tach of the SXC/EXC models, but it won't give you the gauges you are after.

Headlight: stick in a HID conversion kit. Not road legal in most western european countries (UK is a different story), but who cares once you have departed ... Alternatively, rob a bank and spend 350 EUR on the Touratech one. Don't use higher wattage bulbs - stuff will smolder and melt, and the stator is already weak as it is.