I would get a space saver high chair for travel, the kind that fit on a a chair. The clip on high chairs are fine, but they don't always work well on every table.
As for a convertible car seats, they are good from birth (except the Clek Foonf, it's from sit unassisted and up). Installed at an infant angle or more upright? Depends on the seat and lo. Realize that convertibles take up less space front to back than an infant seat (30" limit on your seat? Snugride 30?), so even at an infant angle it will take up less room. To be honest, the Elantra is deceptive, there is a fair bit of space in the back seat, if you can fit the Snugride you should be able to fit whatever seat you choose, this car doesn't seem to have much problem fitting seats. Now it's up to your budget. Good buys? Graco MyRide, Safety 1st Complete Air, Diono Radian, Clek Foonf (in a few months).
Also, I just wanted to point out that the over height limit isn't a hard and fast rule. It's the measurement from the top of the head to the top of the seat, it's usually no more than 1" from the top of the head.