What type of VPS should I get?

40 WP plugins? You are going to need whiskey or Valium. ;)

You will need more than just a powerful droplet, you need optimization. so…

Create a 1gb droplet. Install Nginx/PHP-FPM, Mysql (or Percona if possible), and make sure to use w3 total cache. Make sure you have APC cache enable in php.ini and installed on server. Create a sencond $5 droplet and install varnish4, configure varnish4 to use your first droplet as backend and make some optimizations too (plenty of guides around). Then open a free account aat cloudflare and protect/cache your domain name redirecting it to your varnish4 droplet.

it dhoould looks like: Cloudflare -> Varnish4 -> Nginx.

Also you can add a second Varnish4 droplet and use them as load balancing, remember to create a image from your droplets so if anything fails you have a backup, and if traffic goes up you can always launch an extra instance for load balancing.

If all that becomes difficult for you, try serverpilot.io

Check some guides on php-fpm/nginx optimization too, and make sure to use w3 total cache to use APC as cache for sql queries, also take use of the CDN options and create a CDN using your nginx (virtual host with static files rules), cached by your Varnish and Cloudflare.

As you will be using cache, you have to learn how to create some rules to avoid caching in some parts of your website, specially the transnational pages where you should not cache content.

It could be difficult at first but is a great option to reduce costs and increase performance. Also you will need to test if your site is compatible with caching, ie uses unique url/params for each user/session or so.

Also cloudflare is offering fre SSL for free accounts, so you can have SSL for free on your commerce site which is great. For this you need to create a xinetd.d/httpsd file to allow SSL traffict to redirect from varnish4 droplet to your webserver (as SSL can’t/should not be cached). here the code:

Install xinetd if not installed.

# description: sends ssl packets from 443 to production server 443 port bypassing varnish.
service httpsd
{
 type = UNLISTED
 flags = REUSE
 socket_type = stream
 wait = no
 user = root
 redirect = 2.2.2.2 443
 port = 443
}

Restart xinetd

Of course this is just a suggestion, fell free to consult guides here at DO or Google for some, there are plenty and very good ones, so one should adapt to your needs.