Yeah, kind of a tough choice in some respects. Your current EXT is a good platform, but it does need a couple upgrades when going brushless. Mainly, you'd need the Brushless model diff ($18), the brushless model front bumper ($3) and some Music Wire from your local Ace Hardware for wheel drive pins (1.57mm) and diff bevel gear/cup-retainer (1.19mm) ($3ea for a lifetime supply) and some good gear grease like TrakPower Waterproof Gear Grease ($4). You may also need a new steering servo cuz the OEM unit is pretty weak. Those can be had at HobbyPartz.com for $12-$15 for a metal gear digital servo like this one:
http://www.hobbypartz.com/servo-mg946r.html.
As for the motor system, yeah that DTX Element system should be nice, and should be useable with the Castle-Link dongle ($25 at your LHS) for custom tuning of the ESC, but then, you're into the motor system for $150 at that point. For $85 you can get a great little HobbyWing EZrun system like this:
http://www.hobbypartz.com/ezrun-35a-9t-3650-motor.html and it comes with a programming card. I have two of these systems in both my Evaders and they're great. On-road, the car will do almost 40mph, plenty fast. Off-road the car's a blast to drive. You do have to keep an eye on motor temps, depending on your gearing. I'm assuming you're running the stock gearing 88-tooth spur with a 20-tooth pinion. I've geared mine down a little, using a 19T pinion, and that keeps motor temps in a normal range. But DO get yourself an infrared temp gun. Gotta keep motor temps below 160F. BL motors can run normally up to 150F with no problems, but if you start getting over 160 and pushing 180 you run the risk of damaging the motor windings.
Oh and you'll want some decent tires. The stock ones don't give you much grip off-road. Too hard. I run ProLine Gladiator tires on RPM Slingshot wheels (for the Losi XXXT) on the rear and ProLine Dirt Hawg II's on the front. Wheels are $10/
pr tires are ~$20/
pr. And get some lead tape for balancing the wheels, it'll help keep strainfrom the wheel bearings. The rear bearings take a beating when spinning wheels at 10,000 rpm.
So you see, even with the needed upgrades, you can upgrade your EXT to Brushless spec for less $$ than buying a new BL one. And you get better tires! Good luck, let us know if you've got any more questions.
P.S. You didn't say anything about your radio system. For $25 at HobbyPartz, you can get a good spread-spectrum TX/RX (no crytals) by FlySky.
http://www.hobbypartz.com/fs-g2-24ghz-radio.html