I doubt any school is going to let you do your Skills Test without doing at least one recent solo cross country.
And they aren't going to let you do a solo cross country without doing a cross country with an instructor first of all.
So that's 3-4 hours without any refresher training for basic handling skills, stalls, PFLs and some circuits.
I'd be really surprised if you could finish everything in 6-8 hours after an 18 month break. It might be possible, but probably unlikely.
As for being told exactly how much it's going to cost to finish, that is impossible. Each hour of flight traning is expensive, and nobody knows exactly how many hours it will take to get you back to the stage you were before you stopped flying. Therefore it's not possible to say exactly how much it will cost.
You'll probably get to finish it a bit quicker with your old school, as systems and aircraft will be the same as you were used to. However you should also consider what will happen after completion. Will you continue to rent from them? If not, then you might be better finishing your PPL with whoever you intend to rent from after completion of your PPL, to avoid the need for conversion traning.
dp