To try and address various questions. (
Note: I'm mostly familiar with the 3DS version. Mobile has some differences in things like coin farming and a different event schedule.)
- Ampharos (Stage 130) is usually the best stage to grind on besides Saturday Victini. Ampharos has 50 moves, only surpassed by Snorlax, but it also has relatively tame HP and disruptions compared to Snorlax's massive HP. It's also easy to grind NVE Pokemon as long as you have a Ground type to balance it out.
Buneary is your best bet until you reach there. Generally, a Pokemon awards as much EXP to every Pokemon (not split, apiece) as it allows moves to clear it, so Buneary awards 30, while Ampharos awards 50.
- Base 60 is the minimum to use a Pokemon over something else, though that tends to also be when the mon has a particular ability. If you watch events and such, Base 70 mons and one or two Base 80's will come to you soon. This is of course after noting to use SE mons first and foremost, though a few later stages will be made easier by bringing certain mons, such as Milotic wanting you to bring Feebas.
- Coins can be farmed from Stage 37 (Meowth). Here is a small tutorial video
on how to do it. Farming this stage if you hit a wall or much later when you've cleared everything is a good way to build up coins for upcoming events.
- Are you familiar with the Great Ball trick? Where shutting the game off after using a Great Ball but before going back to the main map gives you back the coins for the Ball (not items though)? It's a nice way to hold onto coins until a successful capture.
Word of warning, you're approaching what many consider the first "roadblock" stage with Mega Mawile, since it has somewhat troublesome disruptions and forces Eevee on you as an extra Support. Main limiting factor would probably be the lack of good Megas since Mawile is Steel type in this game.
Landorus is available right now as an event, with Base 80 Power and Ground typing he's probably be very helpful to you. You get one chance a day and I think this is the last day for him. I would recommend using some items to try and get him.
One thing that will be a big factor in advice,
are you playing Mobile or 3DS?