Technology trading shouldn't be restricted in a way that Shark suggests, only optionally.
Although I agree that if the player aims for a challenging and realistic gameplay, he/she (most likely he) should only use it in desperate situations, when the empire is at stake.
If you read any sci-fi book, or watch a series you can see, that technology trading almost never happens; and when it does the price is unreasonably high, because there's no time or other solution.
Or just take a look at some higher-tech companies.
Btw when starting PreWarp, while other empires are on tech level 2-3, set research to 999k, corruption to very high, then how does technology trading help? Friendly relation is needed for tech-trading. With the above settings probably only Mercantile Guilds have enough money to buy techs, but they rarely get friendly with anybody (especially on unstable/chaos), and probably better off with research boost.
Don't get me wrong, having more options is the best thing that could happen to Distant Worlds, but if tech-trading is an issue for you, there are ways to negate its advantages.
As for making a game challenging (other than the trivial settings) the first step is choosing an inferior race or one that doesn't correspond to your playstyle. I think for most of us both applies to Atuuk, and neither to Ackdarians.
Races with access to Technocracy and Marcantile Guild are easier than others.
I also find Cluster maps to be more difficult.
I get in a cage fight with 1-2 empires as soon as I start expanding, then have to set up bases in other clusters to be able to attack them.