1. Differences training is mandatory, just as it is for, say, a taildragger. No test however.
2. Annual about £150 including inspector and BMAA fees.
3. 3rd party insurance about £80pa, full hull insurance (including 3rd party) generally around 8% of hull value. Insurance is not legally mandatory, but you'll find yourself banned from many airfields if you don't have 3rd party at-least.
4. 4-strokes have a TBO of about 1200hrs, 2-strokes 300hrs. Typical servicing costs
here Personally I think that the 503 is the best of the 2-strokes, and the 912 and Jabiru the best of the 4-strokes.
5. I'd reckon on about 3-4 hours for a 3-axis microlight, 10-12 for a flexwing.
6. As Fly Stimulator says - depreciation is slow and minimal.
7. I find it usually costs me fuel plus same again, that's with 3rd party insurance and reasonably cheap hangerage. So, at 15 litres/hr, it'll cost you around £20-£25/hr all in. Add in extra insurance or expensive hangerage (I'm paying £30/mth).
3-axis in the £5k bracket you're limited. 2-seaters, perhaps a Thruster TST, Spectrum, ropey AX3, MW6. Single seaters, MW5, Goldwing (not an aeroplane for a 100hr pilot to be honest), Sluka, Tiger Cub (very cheap, fairly horrible).
Pick of the bunch for a £5k 3-axis are probably the Spectrum (if you can find one), Thruster, Sluka or MW5 - in roughly descending order of price.
G