I will start with my default profound apologies for the US continuing to be bass-ackwards when it comes to measuring units!
Since we're talking about "threads per inch-or-cm" the conversion is pretty easy as 2.54 is the factor dead-on.
So in our case of talking about a US 135 mesh screen (for powder) we divide by 2.54 and get 53 which is right on for a 55T screen.
The 230 mesh screen we use for enamel would convert to 91 giving us a 90T screen to use. Easy peasy!
I pasted a list of of common conversions below.
One thing to note is that the "T" in metric screen counts indicates medium thread thickness. That would be the most common screen you'd run into but there are also "S" screens (small thread diameter) and "HD" screens (heavy thread diameter). Those would let in respectively more or less ink (larger or smaller particles) at the same thread count.
The US helpfully hides that info but for general use screens you can assume they are effectively "T".
Here's a good resource for way more than you'd ever want to know about screen mesh sizing:http://www.catspitproductionsllc.com/screenmeshconversions.html
Re: the wooden frames my advice would be to look around a little further to find aluminum frames. They are so much more light, durable, and easy to maintain. They might cost a bit more in the short run but I think you'll be much happier with them over time.
We recommend using yellow mesh. It can require some more exposure time but it should lead to a more crisp stencil overall.
Good luck and please share what you discover!
US <> METRIC
38 <> 15T
61 <> 24T
86 <> 34T
110 <> 43T
125 <> 49T
140 <> 55T
158 <> 62T
196 <> 77T
230 <> 90T
255 <> 100T
280 <> 110T
305 <> 120T
355 <> 140T
380 <> 150T