I'm positive anyone who donates is incredibly trustworthy though.
Don't put it past the younger players stealing a parent's credit/debit card just for the sake of getting admin. It'd be best if you took an age limit on giving those more powerful rewards so you wouldn't automate giving a 10 year old kid admin because he got lucky while looking through mummy's purse.
Personally, I'd see the reserved slots as a better reward, the donators only server an even better one. With limited spacing available though, that could become a problem if there are more donators than the maximum slots available, but a donators only server would probably serve to buffer this a fair bit.
I, personally, have never been very keen on the idea of donator admin. I can only speak from a totally different angle here, namely MMO private servers like Maple, but with donator admin abilities, it just seems to feel like admin powers have become a fun gimmick to donate for, rather than actually being something to earn for dedication and loyalty. Plus, as I stated above, it can be very easy for lots abuse should someone get lucky with a credit card.
I can't think of any suggestions to add to the list of possibilities, seeings as we have pretty much all possibilites you can put forward in a Half-Life 2 mod...