Depends how much you have/want to spend in one go.
You've got choice of keeping the HL chassis or upgrading to the mato or Asiatam/Taigan chassis.
If you want to stick with the HL (Mato is same but metal)
I say you best upgrading to the Hentec idler adjuster (£30) as chances are if you stick with the original the metal tracks will eventually rip the idler off the chassis (I done this with pastic tracks

)
Next upgraded metal suspension arms and better springs (£10-15) this will allow you to use the original HL road wheels or upgraded metals ones (but make sure the axles will fit the suspension arms as some will not)
Final get metal tracks, drive sprockets and idler wheel (£50-60)
If your willing to spend a bit of money up front
Asiatam/Taigan chassis (£50) but none of the HL roadwheels or idler wheel will fit (different axle size) so you'll also need metal road wheels (£30-50) and metal Idler wheel (£10)
From there you can run with original drive sprocket and tracks or buy the metal upgrade kit (£50) I think Forgebear has them.
Finally the gearboxes.
The standard HL tiger is way to fast. So any reduction type (3:1 etc) gearbox will bring it down and give the tank more power to turn etc specially now you've added weight with all the metal bits (my tiger weighs 6.5kg!)
There is basically three types of gearboxes available steel, brass, black metal. I've used all three and say steel are fine but very noisy, brass quieter but brass is soft and i've bent the brass drive shafts all to easily. Finally black metal are best of both steel and brass strong and quiet. (ish).
Also with gearboxes 3 types to fit the tank. Long shaft, short shaft and low profile. I think from memory you need long shaft type.
And finally like I said the tank will be heavy with all those upgrades so I would recommend upgrading to a larger capacity battery, something like 3000mA upwards and you'll also need a better quality charger for the bigger battery as the HL one is rubbish and would take 8 hours or something to charge a 3000mA battery.
Hope that hasn't hurt your head to much
Ian.