A bigger SSD will be a good thing, but you need to also solve the issue right now.
You can't delete built-in or DLC content because that's kept in package files. These are all-in-one files that contain not only the routes and sessions, but also the other assets. What you can delete are those things you create, download from the DLS, or install from a 3rd party site such as Jointed Rail.
There are a number of things you can address here without going for the extra expense and deleting routes unless you want to.
The first place I would check is the backups folder which is located where your data is installed. For most people, this is under C:\users\<your login>\App data\local\N3V Games\TANE\Build xxxxxx. The xxxxx refers to some letter-number combination.
The Backups folder is arranged in folders by date and there can be up to 7 backup-days in the backups folder. The current one I have in my backups folder is 21-12-14 and is in YY-MM-DD format.
Every time you open up a route, even to look at it in Surveyor, touch any content, or delete an asset, a backup is made of it. With the size of the assets becoming larger and larger, the space can get eaten up pretty fast!
By default, the setting for the backups is 7-days. You can change this to something less by going to the Launcher, clicking on Trainz settings then the Dev tab.
The folders themselves can be deleted from the backups folder manually and you should do this first before adjusting your settings.
As G.M. mentioned the other place to look is the Internet cache folder. This is located under the cache folder in your user data folder. This folder contains everything you have downloaded and does get purged, but probably not enough which leads to a disk space issue on some systems.
Once you've cleared these locations, check your disk space again. I'm sure you'll be happy. I recommend running a TRIM operation once you clean up your SSD. This helps with the data management and performance on the drive and can be done by running Defrag.
Now regarding out-of-date and obsolete assets.
Out-of-date assets mean there is a new version available for installation. Obsolete means the old one is installed still and can be removed. You may also see obsolete on the DLS. This means that a new asset is available for download and the older version is still there. We can't manage the DLS so unless you need the old version instead, you can ignore that.
Built-in and DLC assets are periodically updated with new versions from the DLS. The space occupied by the old assets does not change because they are installed in compressed files located in sub-folders under packages. Being built-in, or in those compressed files, we can't delete them so ignore them.
When it comes to asset management, we recommend you update the out-of-date assets and delete the obsolete ones. Once you set your backups to a reasonable level, you can worry less about disk space, but periodically go in and clean that up as we all do.