What is a good but affordable torque wrench to get?
Define affordable. You can get a Norbar for about £77 http://www.totalcycling.com/index.php/product/norbar_4_20.html?action=currency&id=GBP , which is the same wrench as the Syntace one I use https://www.starbike.com/php/product_info.php?lang=en&pid=6048
If you can't afford either of those, you probably can't really afford the quality of handlebars which need that level of assembly precision.
@gbj_tester started
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Define affordable. You can get a Norbar for about £77 http://www.totalcycling.com/index.php/product/norbar_4_20.html?action=currency&id=GBP , which is the same wrench as the Syntace one I use https://www.starbike.com/php/product_info.php?lang=en&pid=6048
If you can't afford either of those, you probably can't really afford the quality of handlebars which need that level of assembly precision.