2 Tasks one Big Project

Projects General Guidance
1

I'm in need of 2 things... that may ultimately change depending on if one or the other is created.
First device:

One takes serial data (RS232 at 38400 baud) in from 900 MHz radio. It acts like a wireless serial connection so you could essentially consider it as being connected to the Garmin GPS 16x HVS on the other end. It then parses the incoming NMEA strings and then sends it via ethernet to a server that pushes the data out to a location tracking application. I'm concerned about how fast it can handle this exchange and the required ram so I don't think a standard atmega would be enough. It currently needs to handle about 300 Bytes of data per unit. We currently have 14 units with the hope of adding much more.

The other is a device that again takes in the serial data from a Garmin but is connected directly to the Garmin GPS 16x HVS then sends it out via a RS232 serial connection. (into may radio on the other end) It would also need to be able to handle the data parsing but the difference is it would only have to handle data from the single GPS associated to it.

Any ideas on this? I'd like to create a custom PCB for both of these (to reduce cost and make connecting that much easier) but I'm at a lost for where to start on selecting the required chips/controllers.

2

It currently needs to handle about 300 Bytes of data per unit. We currently have 14 units with the hope of adding much more.

You could use an Arduino Mega 2560
Also is RS232 5v, might need to check.

Send data in your own way, leaving the non necessary and never changing data out of the transmission.
You could store the results to and SD card while processing and maybe even compress it during idle times.

If you get a Mega 2560 you can buy a Quadram thingy and have more RAM than you can poke a stick at.

Shouldn't even worry about a PCB until you have a working design.

3

Also is RS232 5v, might need to check.

It is not it is + / - 12V with a logically inverted signal, so what is a HIGH in TTL terms is -12V in RS232, and a LOW or ground level is +12V.
You need a converter chip like a MAX202 or one of the many similar offerings.

It currently needs to handle about 300 Bytes of data per unit

But how often does it need to do this? Once a second would seem reasonable, this is well within the capability of an Arduino.