Installing condensing boiler

We are having our existing LPG boiler replaced with an oil burner. Ideally I'd like a condensing boiler however there are 2 potential issues:
The boiler is below ground - presumably this can be remedied via condensate pump.
The current set up does not have a balanced flue - could the existing set up be used, ie exhaust is connected into a chimney stack used soley for the boiler and there is a separate plastic pipe leading to the outside wall at ground level for air intake.
I have had a boiler engineer quote me but he said nothing of the fact that the flue was non-balanced and I'm just concerned that nothing is missed and I end having problems down the line (eg selling the house etc)

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