The SamsungGalaxyAce doesn't let me download apps!Even with a huge SD Card
If the device info is correct here then your device's internal memory is 158MB (quite low IMO) in total out of which you can use Diskinfo(see Userdata in it) and Diskusage (both &lt;1MB individually)to see how much space is used and is available. You can then uninstall any app that you may have installed at some time but don't remember. 

However, I don't see much scope here since the issue would arise again sooner or later considering the (very low) storage set by OEM. 

Note that the storage used by apps is /data which is different from the external SD card that you've of any size. Some apps until Android 4.2+ can transfer data files (.obb) into external SD card rather easily but it is a painful job in above versions now.