Head gasket is a given - but do get a decent head-gasket. I'd suggest Elring. Whilst the head is off get it skimmed to true and do the other head work mentioned
I don't see the need for a bigger turbo to honest - but I do know of people who have done it. It depends on just what power you need (note 'need' not 'want')
I run a '93 Discovery 200tdi, with 111k on the clock. It is over maintained (example: I have a clutch hydraulic issue at the moment, so will replace Master cylinder, slave, damper and flexi pipe at the same time; all new parts and all OE or better (so AP Lockheed or Lucas)
At 90k the head was removed and to quote:
"...has a Hamlin's stage 2 head, with matched inlet and outlet ports. the inter-cooler inlet and outlet and inlet manifold have been ported
The engine has now done 110,000, with 20 odd k since the top end rebuild.
The pump has been tweaked to match the work, but no ring or pin upgrades.
It's rolling road figures are 131bhp@4250rpm and 242 ft/lb@1800rpm
I'm particularly happy with this and the 35mpg I get at 65-70mph on the motorway, 33mpg around town. Towing sees very little drop, to about 32mpg.
I run Shell Rimula 15/40 Non Synthetic, with 5-6k changes. The right oil makes huge differences in a 200tdi"
At the same time as the porting, the valves were reground to match re profiled seats. It runs a standard, but ported intercooler and the inlet manifold has been ported on it's inlet. Note that the fuel consumption is UK gallons, not US
I wanted reliability, with the option to tow, comfortably up to all up weight 3,500 kgs (7,700lbs), carry my family at modern day traffic speeds in Europe (so 75mph) and get sensible fuel consumption. Tuning a 200 is often a difficult balance of 'robbing Peter to pay Paul' so you lose bottom end grunt in favour of top end (made worse by using a larger turbo and intercooler). I don't run huge tyres, so my gearing is near enough factory. I am going to fit a slightly bigger intercooler in the future - but only to reduce EGT.
In the UK, changing engines is a pita. It effects insurance in a big way - if you want to stay legal. It's also a pita when you travel abroad, as engine numbers and spec need to match (in some countries) and it's only when you've actually experienced the resultant problems that you know what it's like.
Additionally I like to keep parts commonality simple - LR parts are not expensive and are easy to get, even in remote places. I like that the 200tdi will run on pretty much anything that will compression combust.
More power is always nice, but generally it's a want, not a need; and it often comes at the expense of unreliability
This is my 11th 200tdi powered vehicle in 29 years. So I'm about getting the hang of them