Just bought HJ60 factory turbo.... Any tips?

Hi everyone, I have lots of questions... Just today I picked up my new beast, an 86 model HJ60 with 12H-T engine, and five speed... It has done 350k, the tyres are crappy Federal H/T which will be gone this time next week upgrading to a better A/T tyre in either 32 or 33 inch... And am still deciding who to give my money to re the saggy suspension... Any advice on tyres that anyone has used to good effect on same type of wagon and suspension shops in Sydney will be greatly appreciated... But to my main points.... At around 100kmh it has this kind of vibration that comes and goes in a pattern type. I will be driving at 100kmh and a vibration seems to come (not seemingly from front tyres, feels more like just behind the front wheels... gearbox?...:(...) and stays for about 3 seconds and then goes away for about 5 seconds and it just repeats all the while I am at that speed and disappears at about 80-90... Any ideas??

Also can anyone recommend to me a Toyota turbo diesel guru in Sydney? I have heard that the guys in Berrima are very good but I would like someone closer to home for tuning and the more involved maintainance that the car may need. I live near the airport. Can anyone from experience tell me the best type of engine oil to use?

Any advice on any of the above topics or anything else regarding my type of vehicle will be warmly welcomed... Many thanks