Hi roadwarrior,
Are you planning to go off-road trailing with the vehicle or just use it as a workhorse?
In any case, being an LN166 owner myself, here are my answers to your questions.
1. is it worth it at 675k
From the looks of it, the asking price seems fair. How may KMs has it run so far? Get a complete inventory of upgrades and accessories on the rig. How has it been maintained? Get the maintenance records if you can.
2. if i want to replace the tires, what are ideal tires for this rig and how much should i expect to spent.
This will again depend on what you intend to do with your rig. What kind of rims are on it now? If 15x7, then you can go with A/T tires in the 31x10.5xR15 size for a balance of height and better economy (compared to the existing tires anyways)
3. is it practical to buy this rather than the newer strada or hilux model.
Armed with the knowledge of what you intend to use it for, only you can make that assessment. But if you want to minimize cash out for a trail rig, this could be a good start. You can learn stuff about your rig and diesels in general (presuming this will be your first diesel vehicle) then when you feel you are ready and think you will be happier with a newer model, you can buy a new unit.
4. how could i make it accelerate faster?
Change the tires for one thing. Diesels don't really accelerate fast so if your point of reference is a gas sedan, the difference is kinda night and day. You can install a turbo kit.
5. can i add turbo on this model?
Yes, you can. This mod is actually on my list of to dos on my rig. You can either install a kit pulled of a 2L-T engine or go the "official" route and spend for a turbo kit specifically for the 3L engine.
Hope this information helps you.
If you need any more assistance, don't hesitate to ask.