I've been collecting for 35 years & have always focused on my core of RNZAF though over the years, like many others, i've branched off as interesting groups present themselves. U-Boat, Luft Paratrooper, Glider Pilot Regiment, Heer Pz uniform & insignia haved passed through my hands but i've managed to keep my collection of Heer, Luft & WSS assault gunner uniform.
And like most collectors who have responded to this posting, i get much more enjoyment from a focused collection, branching off causes you to take your eye off the ball & qualitity peices get lost amongst the oddments.
So stay focused & shed a bit a bit of gear, you won't regret it!!