canon 400mm 5.6L for 500$. I figure I could just but a 1.4 teleconverter on the 400mm if need be instead of getting the 150-600mm sigma. What do you all think about that?
I use a 1.4 TC on a new Canon 100-400 L and it works well BUT keep in mind that it becomes F8 and the auto focus works better with more light. I'm actually going to get an R6 for this reason as I have a 2x TC as well making it F 11. The R6 can auto focus with less light than that, I believe down to F 16.
You can't go wrong with great glass and if it is in good shape I would do it but you will likely be upgrading from the T5 next. I had one, but ironically as I got better glass I outgrew it. Used 5D Mark IIIs may be a real bargain this fall or if you want a crop sensor have a look at a used 7D Mark II, but I think they may hold their value better until Canon releases a true pro grade mirrorless crop sensor body.