Where in the world are you? When was the last time you saw the garden before moving in? If this is a temperate zone mulberry climate, Do Not Do Anything until middle to late spring.

Is there nothing in your purchase contract that says the property should have been in the same state on the possession date as on the date you viewed it? This is a normal part of a standard real estate contract in Canada and prevents this situation from happening. If you do have this, you can make the previous owner replace all the plants. It may or may not be worth it, depending on exactly how much and the types of plants removed.

What a great opportunity to learn how to propagate various kinds of plants. It's not difficult and it's fun. You can get all the plants your want for more or less free, with a little time and effort. Spend the winter developing a planting plan on paper. It will give you some direction for what plants you need to be scouting for in your neighborhood/town and possibly Ebay. Seeds are a particularly great way to get a massive amount of plants for almost nothing

Unless your contract specified that the seller could remove all the plants---he couldn't legally. It's like removing all the light fixtures. Whether it's worth pursuing is another matter.