Need Advice On Ageing A Single Malt Whiskey In Own Barrel

[Pauli]

Hi , I have had a 50l American oak barrel made by a local cooper , he has toasted the inside of it for me.
I want to now fill it with a single malt whiskey to let it age with me so that over next 40 years or so (god willing) I end up with something to enjoy and share with friends and family.
As you will se by my questions I am a novice and have various opinions as to what I should be doing.
I have been told I should fill the barrel with a sherry for some period of 3 months or so ,before filling it with whiskey?

I wonder what would be a good whiskey to fill it with.?
( would prefer to start with something that may be reasonable cost per litre ,with the hope it ages into something far more valuable ..thinking along the line of a Glenffidich 12 yo , as I see the 18 yr old , 50 yo are more expensive ,so I assume better.
Open to all opinions.
Cheers Pauli
(Australia )