The key differences between the R&S IN 115 bespoke PS and a lashed-up solution of the sort you propose are the following:
"interferencefree" versus what is very likely to be a noise-generating switching power supply.
Built-in wide bandwidth power injector.
"short-circuit-proof" versus probably not. Low current limits (200-500mA) versus probably too high.
You should probably use a (quiet) linear PS with a reasonable low output current limit. A switching PS with noise and probably too high a current limit is less than ideal.