If you're going for all the other achievements in the game, then you can get this achievement, without extra grinding, with enough spare money to get extra suits and anything else you'll need for the other achievements.
You do this by going for fully completing all the championships and by following some simple tips as you do so.
Your basic aim is to get as much money as you can from every opportunity. You want to have 14 million credits by the end of the game so you can buy 700 copies of a car that costs 20k credits (there are two or three that do so - 20k is the cheapest). The 700 cars do not have to be all different.
Although, in reality, you'll probably have about 100 cars from just playing the game at this point, so you won't need quite as much.
To gain this amount of money without having to grind, do the following:
1. Never ever buy a car unless you need to for a competition
Any cars you buy are slowing you getting to the 14 million target. When you do need to buy a car, also make sure you buy the cheapest one possible.
2. Be sure to make use of mods
This is massively important. Buy the premier mod pack (35k each) to get the best mods, so that you can equip them, and gain more money per race. This should net you 20-30k a race (when I did all this, the mod pack cost 50k and I still was in profit, so you'll manage it even better; I bought 10 mod packs at a time so I could use them efficiently - three mods that are all the same so you don't have to do the same things multiple times etc).
3. Only buy mod loot crates, and the premier ones at that
Don't waste your money on any other type of loot crate.
4. Try to race using as few aids as you can, and with a drivatar difficulty of Highly Skilled at least
Sometimes it won't be possible, as the game just makes it overly hard on some events/circuits, but raise it back when you can, as racing with as few aids as possible and on as high a difficulty as possible nets you extra cash each race. It all adds up!
5. Do the showcase events as soon as you can
This is because they give you a free car when won, which means it's one less to buy for some of the tournaments you have to enter.
6. Complete championships asap too
For the same reason as above, complete entire championships as soon as you can too, as that gives you a free car.
7. Complete the weekly Forzathon challenges
These also grant you free cars, as well money bonuses and driver gear...all of which will help with this and/or the other achievements.
8. Let the freebie cars you get (from points 5, 6 and 7) level up your car collector level
Don't be tempted to buy any to increase your collector level; while you do lose out on end of level cash by being a lower level, you'll soon get to the levels that give you 50k at each level up from the cars you win from showcases/championships.
It'll also be impossible to enter some events without having to buy a car, so leave buying to when you really have no choice.
9. Always choose the money option when you level up
You may think that it's better to get the free/discounted car, but it isn't, because each 20k you get is an extra car at the end of the game...you'll get 20k for just levelling up, and once you're a few car collector levels in, you'll be gaining an extra 50k on top of this.
Having said the above, if the car in question is one that you'd need for one of the tournaments, then do choose the free/discounted car option, as it'll save you money due to you needing it anyway (you can click B to make the decision later if you want to go and check car requirements for specific tournaments, before going to the Prizes section of the main menu to finally make the choice).
However, it should be noted that there's a chance you'd get a car in the same class/bracket for the tournament from winning an entire tournament or showcase or from completing Forzathon challenges. I think I choose the free/discounted car option about 5 times max, and most, if not all, of them were towards the start of the game before I had formulated this plan...so it isn't something to worry too much over.
matthias005 has also added that, if the car you get for a level up bonus is a popular car, such as a Nissan GT-R 2017, it will sell very well at auction - way more than 50K. So, while it may be hard to know what cars will sell and what cards won't, be on the lookout for anything obvious that may well sell for more than 50K at auction (matthaias005 said he got upwards of 250K for the GT-R!).
10. Always get the weekly payout from the Forza HUB
This nets you around 250k for no effort at all (usually refreshes on a Friday, but check often regardless). The Forza HUB can be accessed from the main menu (just go to the daily rewards section once in the hub, and you'll automatically be allocated the money, which you then have to download from the gifts section of your mailbox)
Using these techniques, you'll find you'll get to 14 million credits before you finish all the championships, at which point you can start treating yourself to the odd car and other things, but you must be sure you have enough credits for when you're ready to buy the 700 cars at 20k each.
The easiest way to buy the cars is from the car collection screen, where it shows all your cars and their levels on one scrollable screen from the main menu. You can basically one-click buy each car, so you just keep hammering the A button without having to pick a car colour or design etc.
You can sell the cars after you've bought them to get half the money back; however, I would be wary of doing so, because if any DLC has an achievement for having 800 cars, this will be very hard to do so if you've sold them all.
This is the technique I used, when the premier mod crates cost 50k each, and Forzathon only being introduced towards the last week of me playing. Despite all this, I still ended the game with around 3 million credits left after buying 600 20k cars, which I spent on getting the extra outfits from look crates to finish those achievements off.
So it definitely works and eliminates any need for grinding (other than grinding through the game itself!)