The 100 doesn't come with infant insert and they don't seem to show it available for sale on their web site. You could call and ask if they sell it separately to use on the 100. But looks like you need to buy the 120 or RXT.
How long will you be driving your Echo? It's a small car and I'm not sure the Radian will fit without the angle adjuster; which you can't use until baby is a year old. My parents had a Yaris sedan, which supposedly had more cabin space than an Echo, and the front passenger seat was so far up that my mom's knees were pressed against the dash and she's only 5'2".
We used the TF instead in their vehicle and had it installed as upright as allowed, and she fit, but I thought she was unsafely close to the dash.
In your situation, I might go with the MyRide as it fits really nice in small spaces. Amazon had it last week for $150, but looks like it went up again to $169. You could use it until your son outgrows it rear-facing and evaluate then if you need to get another convertible or are fine with moving him FF at whatever age that is. At that point you could always sell the MR to fund a taller seat if necessary.
An FYI, my 2nd child was my largest infant and she was somewhere around 85th percentile for height and weight. She outgrew the MR at 4y8m by weight with a half inch of growing room left. Her height had slowed down and her tall torso also grew minimally while her legs sprung. So you never know how long that seat will fit!