Always, always run test, either by itself, or as a base. 48 is when my natural prediction dropped (I had been natural for 5 years) and I run 350/week year around unless prepping for a show (I am nationally qualified to compete for a pro card). I run low dose deca (150-200 per week) about 6 months out of the year to heal up injuries and keep me training. As you get older problems with deca increase. We know that "deca dick" is caused primarily by prolactin, but prolactin can be treated by an inhibitor like cabergoline or pramipexel. However, research is emerging that strongly suggests that there is something besides prolactin that causes deca dick. Again, this is a problem more for older guys than younger, but something to watch. My approach to deca is "as needed."
My take on deca is I use it to foster pain free training, so I kind of need it. Primo is great, but I don't "need" it and it is expensive. In any case, a primo or deca only program is and would be a huge mistake. Just go with test. More importantly, find a knowledgeable doctor. Run labs, measure your total T, SHBG, and free T. Run all your general health labs as well. One of the top TRT docs in the country (I send my clients to him) has a ton of older guys running 300-350 test/week year round and they are very healthy.