Personally I've made maps with just a lazy grid in excel before, literally just to show placement of units. It's not as pretty, but it's functional. If you've already got the written component (particularly in advanced) explaining what the battleground looks like, dangers ect, to look out for, you really only need a couple shapes on a grid to represent the mechanical stuff. No photoshop skills required. There are alot of programmes and websites you could use to make something simple and straightforward if you need to. People have a great imagination and I would actually say a purely functional map inspires the players to absorb all the scenary details from the posts better anyway. But maybe that's just my weird brain ;D

However it sounds to me you are already comfortable with the idea of roll20, which is honestly a great platform with alot of utility. I would suggest running the written element of the RP on RPGuild and players can jump into the Roll20 room when they need to do mechanic-based stuff. Limit the use of Roll20 to only being a resource for precision and the focus to still be on the rp itself, for example. 

Personally the issue I would have as a GM is having players jump into the roll20 battle without a mediator and making mistakes and whether that would affect the roleplay flow (i.e writing a long post only to find that the situation they thought happened is moot). Do you as a GM always want to be there when they're in the battle map? It may not be a problem at all of course, but something I would be musing with. You could have certain times to use the battlemap or set a rule that players must drop you a message first so you know what's going on before they post. Then again, you might have players who pick this stuff up quickly which would be amazing for the RP flow. 

One last thing I would say, maybe talking to the players (if you have some in mind, or maybe later if you're still unsure) and seeing their preferences on the issue could help.