OK I try to answer But remember I'm a Newbie
You will need a Level converter for the RS-232 (PC) Test. If you don't use one you may
fry your Flow Meter or Arduino.
I agree with your methodology decoding with the PC first.
As far as the Fuel sensor to the Arduino 0-3 volts should work (Standard TTL Levels).
If I remember back from years ago < .7 Volts is a zero (low). > 3 Volts is a one (high).
If you O'scope is correct you may be in a gray zone. Just be aware. These level issues wont fry anything
As far as the Baud rate to 250K Bits. I'm sure google can help.
But since you have an O'socpe available you can use the time period of say 56K Baud and mathematically compute the Baud of you single from the flow meter.
In this forum some users were saying the Aruduino can do 1 mbs to 2 mbs thoughput.
If true that is awsome.
If anyone has guidance on these issues please chime in. I'm learning too.