I bought a 1968 7ECA as my first airplane about 8 years ago. Like you, I wanted to do aerobatices. It turned out to be an excellent choice. They're great planes. I'll list some of things I found as a buyer, owner and seller of a citabria.
The only draw back of the ECA over the 150HP models is climb rate. Cruise speed doesn't change too much. I figured on 100MPH at 6GPH. Fuel tanks - I think the maximum is 18 gallons per side. Mine had 12 or 13 gallons per side which, at 6GPH, is plenty of fuel. If you get one with the bigger motor, you'll want the bigger tanks. If you get an ECA, either size tank is fine. Some of the older ones like mine had a tall instrument panel but most have the lower panel. It doesn't make much difference but given a choice, I'd get the lower panel. Seats - make sure you get an adjustable front seat. It really makes a difference. Don't worry about getting a wide rear seat or narrow rear seat. I never understood the rational for having an extra wide rear seat. Some have them some don't - either is fine. Heater - if you live where it gets cold try to find one with rear seat heat. It's not great but it's better than nothing. Brakes, depending on the age, you may get heal or toe brakes. Don't worry about it. Heal brakes are easy to get used to. Cowling - the older ones like mine have a hinged cowling which I prefer. The newer ones have a one piece cowling. Now the big one - wings.
If I were shopping now, I would try very hard to get a plane with factory metal wings. Mine had the wood spars and I never had a problem but .... Everyone that called when I was selling it asked first about the wings. As time goes by, the number of citabrias and decathlons in the fleet with metal wings goes up. Eventually, as a seller, you'll have a hard time selling one with wood wings. I think the wood wing AD is a bunch of nonsense but it's here to stay. Of course, if you buy a wood winged bird cheap enough it might be worth it. If however a cracked spar is discovered, it's serious money for a fix. You can replace the cracked spar with another wooden spar or go with metal wings. Neither is cheap. I think the metal wings are now in the $25k range - anybody have pricing on them? There are many older planes with that have been changed over to metal wings. All the planes made since around 1993 have the metal wings. The factory metal wings also get a 100lb gross weight increase. That doesn't mean too much with an ECA but it's a big plus on a GCBC or GCAA. There's a third option. A company called millman offered metal spars that can be used with the older wings in place of the wood spars. They don't offer the gross weight increase.
Sorry for being long winded.