It costs a fair bit to upgrade a HL to attain reasonable IR battling standards. Disclaimer - I have picked shops that are known to me and the community, not the absolute cheapest. No bias to pick the most expensive source was intended.

Basic kit (since you're throwing out most everything) - $200 from IMEX

Lower hull upgrades:
DKLM full lower hull - $560,
OR:
RCtank.de track tensioner $50, upgraded gearboxes (PDSGB $165 or IMEX $70), metal tracks $80 (DKLM), metal roadwheels and suspension arms (Aliexpress) $100
So say, between $300-560 on lower hull upgrades
Electronics:
Clark TK22/60 board and miscellaneous items - $120-400

Recoil:
Clark/DKLM - $35-65

Of course, if you aren't battling, you're free to take the à la carte option. But assuming battling is the requirement, the costs quickly start to mount as the minimum money needed to get it rolling would be around $655. And that's not considering shipping costs from multiple suppliers.

To be sure, the Tamiya remains more expensive at $900 before shipping from Danville. For the ~$250 difference in cost, I guess people are expecting big things. For me, the big things are:

• A better, more polished turbine soundtrack (although Clark assures me that later versions of his boards no longer have the horrid stepped engine sounds)
• Y-axis gun stabilizer and turret heading hold (Clark offers this, but you have to buy the (much) more expensive board)
• Fibre optic lighting system
• Designed for extended use, hence maintenance access is taken into consideration (e.g. concealed switches)
• Photo-etched detail grilles

The intention is not to unequivocally state that Tamiya is better than Heng Long - for the extra money it had better be. But recognize that your minimum entry into the Tamiya tank is $900 or nothing. Its perfectly OK if you have the cash, and your requirements for the tank meet or exceed what the Tamiya delivers. But for many guys who don't have the budget, or just do some backyard bashing, or rough terrain running, yeah the Heng Long is good enough. Just upgrade things when (not IF) they break.

I did an experiment once - bought a HL Challenger 2 and did nothing else aside from install a Clark TK22 system to make it IR battle ready. That meant plastic gearboxes, plastic tracks, the whole lot. It was unbelievable how such a large tank could be so light and moved so nimbly! That only lasted one afternoon of IR battling though, because towards the end of the day, one of the tracks had stretched, and both poor plastic gearboxes' clutches were giving way.