Steering wobble could be wheel balancing, panhard rod bushes, swivel pre-load or steering damper (or combination of the above)
Wheel balancing a cheap fix (and worth checking anyway) the rest can be either checked or fixed by a confident DIYer..
If you change any bushes go for either proper OEM metalastic or good quality polyurethane, and change the bolts aswell. (don’t buy cheap bushes from Ebay – they are crap)
As for the 300TDi vs Td5 debate – both have their pros and cons…
300TDi is easy to work on for the DIY mechanic, fairly tunable and strong/reliable if looked after.
It should be good for 250k + if well serviced (my last one died at 145k after a catastrophic head gasget failure – which lead to a cracked block)
Td5 is a good engine, electrics are fairly basic, and I belive the only thing that can completely kill it dead is failure of the crank position sensor.
However, they do drink a lot (even worse if you tune it) but can be tuned to 200bhp+ (they make a great noise)
I’d be looknig at the vehicle as a whole rather than just the engine if it were me….