Look I'm clearly bias considering I own an 87 Tecate, but I think for the same money I would got for the Tecate without hesitation. The example you put up is a very nice example, in fact, they don't often come along as nice as that. The rarity of the Tecate means it will always have some intrinsic value over the more common 84 250R. I agree that the 84 R will be easier to get parts for, but the 85 Tecate is much easier to source parts for than an 87 Tecate, so I can't see the drama personally. The fuel tank and the rear fender both look in great condition and thats half the battle with the Tecate's.
If your a petrol head, then You'll be please to know that in a drag race (stock for stock) the Tecate will smash an 84 250R, and the steering on the Tecate is sharp as a tack and was always one of its strong points. Admittedly the 250R will probably be the more reliable of the two, and dare I say it a better trail machine. But even saying that, the Tecate isn't overly complicated. Make sure the stator, flywheel and coil are all in good working order and you shouldn't have too many dramas with the old girl.
From My point of view, I would give my left nut for a Tecate in that condition. The 85 was the second Tecate that Kawasaki made, and is an improvement on their first attempt (which was the 84). The Tecate was the reason the 85 ATC250R was such a good machine. It woke honda up, as the Tecate was a real demon on the race track, they could no longer spit out rehash designs for their flagship machine that were conceived in the late seventies.
I look at the 84R as the last of the Hondas that were designed without the strong competition of Kawasaki or Yamaha. To that end they are reliable, they go well, and they're comfortable. Unfortunately they will always be considered the poor cousin to the impressive 85 and 86 250R's in my opinion. I suspect that others would share this view.
My toys 85 KXT250A2, 85 ATC250R, 85 Tri-z 250, 06 LT-R450, 04 YFZ450S, 07 125 typhoon x 3, 06 FPV GT.