All 8 plugs the same but front and rear coil packs not interchangeable ( rear are shorter ) change the plugs first before getting too complicated as this usually clears the problem, I have changed 100's of spark plugs over the years and never used a torque wrench, with new plug just hand tight and whatever part of a turn it says on the box ( 1/2 to 2/3 turn) to crush the washer the right amount, if you refit an old plug with a washer already crushed ( why would you do that ? ) it is just hand tight and eight of a turn.
If you use copper grease on thread of standard plugs you need to reduce torque 20% anyway, I never use copper grease on iridium plugs as they have a special tri-valent coating on the thread that does not bond with aluminium head. Standard plugs have a passivated zinc coating that can stick if left in too long, if the old plugs don't want to come out I use the two steps forward one step back with some penetrating oil allowed to soak in ( not WD40) turn forward a bit and then back a bit and easy does it, the steel wall of a plug is surprisingly thin where thread meets main body and you do not want to sheer it off...
I always remove plugs when engine is warm as the aluminium of cylinder head expands more than steel of plug body and this eases the side pressure on the thread, and if you need penetrating oil the heat helps it to get into thread.
http://www.ngk-sparkplugs.jp/english/techinfo/qa/q18/index.htmlOther than that, change all oil and filters and coolant, replace brake fluid and inspect brake pads, discs and callipers the rear callipers are especially prone to sticking on the slide pins especially if gaiters split or not fitted properly. Don't use normal petroleum based grease on slide pins as there are seals on them that will swell and will also attack rubber gaiters, I have used TRW PFG110 for quite a while now and apart from a quick check and another smear when I change pads or discs never had a calliper go tight or seize.
https://www.amazon.co.uk/TRW-PFG110-Grease/dp/B00H8ZX4F8Gearbox oil ( manual transmission ) is every 70,000 miles use a good quality synthetic oil, have you got any whining noises from gearbox in lower gears when accelerating ? There is a bearing in there that is prone to getting noisy, plenty of info on forum about it.
Rear brake pipes prone to rust where they go unto rear torsion beam, worth a check as MOT failure ( but better an MOT failure than a brake failure).