I don't know but I will get most of the games they have revealed so far.

There's Kirby's Adventure, Mike Tyson's Punchout, FireRed, Wizards and Warriors III, Shatterhand, Double Dragon, or SMB/Duck Hunt!

But there's not a high chance of most of those games coming over.

The Minish Cap is the only one I really want.

The Minish Cap would be great. I didn't get it for the GBA back then and it would be a great opportunity to finally play it.

Sooo...

For NES:
Battletoads
Castlevania
Zelda II
A Boy and his Blob: Trouble in Blobolonia
Metal Gear

For GBA:
Mario Party Advance
Golden Sun
aaaaand...I don't know.

Mother 1 aka Earthbound Zero (NES)
Castlevania III (NES)
Metal Gear 2: Solid Snake (NES)
Castlevania: Aria of Sorrow (GBA)
Metriod: Zero Mission (GBA)
Tactics Ogre: Knight of Lodis (GBA)

can someone set me straight on a couple of things...

1. These 20 free games, is it gonna be the first 20 available or can i wait until the 21st 22ed game to use as my free download? Eg If i dont want Balloon Fight but instead want the 11th NES game released on the eshop will that work or am I forced to download these '20' games and othing else.

2. Is there a time limit on this? Do I have to download the 20 games by a certain date or miss out completly?

I'm pretty sure that the 20 titles are determined by Nintendo and you can't choose.
And I think that there won't be a time limit.