It's good that you're thinking about harness height in a convertible seat. That's really important since your tall guy will likely outgrow a seat by height before weight.
I would not get any of the 3-in-1 seats for him. Most kids will outgrow them by height long before they are ready to use the seat in booster mode. Yes, it's a seat that goes to 100 pounds, but the harness can only be used to a certain height/weight, so you'll end up with an outgrown seat when he is probably under 3 years old. Then you'll have to buy another harnessed seat because he won't be ready (as far as maturity and the weight/height requirements) to ride safely in a booster. And it is so much safer to keep him in a 5-point harness for as long as possible.
I would suggest the Fisher Price Safe Voyage Deluxe ($129 on sale now online at Albeebaby.com). They are made by Britax but sold under the FP name. They are the same shell as the Britax, but they don't have all the frills (plush cover, extra padding, built-in lockoffs). We have a Britax Marathon in our main car and a Safe Voyage Deluxe in our 2nd car. My 12-month-old son actually prefers the Safe Voyage. This seat rear faces to 33 pounds and front faces to 55 pounds. It will probably take your child to 5 years old, at which point he will most likely be ready for a booster.
You can also find the seat at Burlington Coat Factory. Since it's the same shell as the Britax seats, you can go to Babies R Us and try a Britax in your car to make sure it installs well, then you could order it online if you like it.
I think you could keep your baby bucket to use in the grocery store (when baby isn't in the car) until he has more head control--as your son is still comfortable in it. I'm not sure about that, though. I always used a baby Bjorn. Just please do not put the carrier on the top of the shopping cart. Car seats were not made for this. It's really dangerous. I've heard that car seat manufacturers are going to start making the seats differently so people can't do this. Always put the carrier in the bottom part of the cart instead.
There are other lower cost options for convertible seats with higher harnesses and taller backs. They won't last till 5 or 6 years, but they are still great. One is the Scenera ($40, I think) at Walmart (the 5-pt. harness, not the overhead shield) and the Uptown (also at Walmart) is $99 and more padded. I think these both rear face to 35 pounds and front face to 40 pounds.
The car seat safety board on BabyCenter.com is great. The ladies on there are certified car seat safety technicians. If you tell them the type of car you have, your budget, and your son's measurements, they can recommend some good seats for you.