SLAR on an LO bird is not impossible. TACIT BLUE showed that it could be done as it was a testbed for the Assault Breaker radar system that later developed into JSTARS. I also know of some LO recce drone concepts that carried both emitting and passive sensor systems.
Plus, of course, there are radars that can know a LO aircraft is out there, but can't target it. In an environment where they know you're there, just not exactly where, you can radiate in small doses and on frequencies that they might not be monitoring and still stay hidden. After all, the new LINK 16 communication system radiates constantly but all over the spectrum such that you can't get a fix on it.
But, certainly, planning on doing primarily passive optical recce and SIGINT is a very good plan.
Programs like MS Paint and the like can produce perfectly adequate graphics for decals, as long as you're willing to put in the occasionally excruciatingly frustrating time to do anything moderately complex in some of those programs.
May I suggest "Paint.net" as a free bitmap graphics and photo editing program? Easy to use, but has some advanced features such as layers and some filter techniques that make it pretty powerful for day-to-day editing and creation functions. For simple graphic shapes and the like, it a good choice.
Paul