diff --git a/ubuntu/18.04/build-disk.sh b/ubuntu/18.04/build-disk.sh
index d5852a5..7068201 100755
--- a/ubuntu/18.04/build-disk.sh
+++ b/ubuntu/18.04/build-disk.sh
@@ -4,6 +4,14 @@ set -e
 # lookup specific binaries
 : "${BIN_QEMU_IMG:=$(type -P qemu-img)}"
 : "${BIN_KVM:=$(type -P kvm)}"
+if [ -z "$BIN_QEMU_IMG" ]; then
+    echo qemu-img not found
+    exit 2
+fi
+if [ -z "$BIN_KVM" ]; then
+    echo kvm not found
+    exit 2
+fi
 
 # get parameters
 RAM_SIZE=${1:-"2048"}
diff --git a/ubuntu/18.04/build-iso.sh b/ubuntu/18.04/build-iso.sh
index bd1968b..64a3f0c 100755
--- a/ubuntu/18.04/build-iso.sh
+++ b/ubuntu/18.04/build-iso.sh
@@ -4,11 +4,41 @@ set -e
 # lookup specific binaries
 : "${BIN_7Z:=$(type -P 7z)}"
 : "${BIN_XORRISO:=$(type -P xorriso)}"
+: "${BIN_DOS2UNIX:=$(type -P dos2unix)}"
+: "${BIN_WGET:=$(type -P wget)}"
+: "${HYBRID:=$(echo /usr/lib/ISOLINUX/isohdpfx.bin)}"
+if [ -z "$BIN_WGET" ]; then
+    echo wget not found
+    exit 2
+fi
+if [ -z "$BIN_XORRISO" ]; then
+    echo xorriso not found
+    exit 2
+fi
+if [ -z "$BIN_DOS2UNIX" ]; then
+    echo dos2unix not found
+    exit 2
+fi
+if [ -z "$BIN_7Z" ]; then
+    echo 7z not found
+    exit 2
+fi
+if [ \! -f "$HYBRID" ]; then
+    echo Please install isolinux
+    exit 2
+fi
+
 
 # get parameters
 SSH_PUBLIC_KEY_FILE=${1:-"$HOME/.ssh/id_rsa.pub"}
 TARGET_ISO=${2:-"`pwd`/ubuntu-18.04-netboot-amd64-unattended.iso"}
 
+# check if target iso exists
+if [ -f "$TARGET_ISO" ]; then
+    echo "Target ISO ($TARGET_ISO) already exists."
+    exit
+fi
+
 # check if ssh key exists
 if [ ! -f "$SSH_PUBLIC_KEY_FILE" ];
 then
@@ -23,15 +53,19 @@ TMP_DOWNLOAD_DIR="`mktemp -d`"
 TMP_DISC_DIR="`mktemp -d`"
 TMP_INITRD_DIR="`mktemp -d`"
 
-# download and extract netboot iso
-SOURCE_ISO_URL="http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso"
-cd "$TMP_DOWNLOAD_DIR"
-wget -4 "$SOURCE_ISO_URL" -O "./netboot.iso"
-"$BIN_7Z" x "./netboot.iso" "-o$TMP_DISC_DIR"
+if [ ! -f "$SCRIPT_DIR/netboot.iso" ]; then
+    # download and extract netboot iso
+    SOURCE_ISO_URL="http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso"
+    cd "$TMP_DOWNLOAD_DIR"
+    "$BIN_WGET" -4 "$SOURCE_ISO_URL" -O "./netboot.iso"
+    mv "$TMP_DOWNLOAD_DIR/netboot.iso" "$SCRIPT_DIR"
+fi
+
+"$BIN_7Z" x "$SCRIPT_DIR/netboot.iso" "-o$TMP_DISC_DIR"
 
 # patch boot menu
 cd "$TMP_DISC_DIR"
-dos2unix "./isolinux.cfg"
+"$BIN_DOS2UNIX" "./isolinux.cfg"
 patch -p1 -i "$SCRIPT_DIR/custom/boot-menu.patch"
 
 # prepare assets
@@ -51,7 +85,7 @@ cat "./initrd" | gzip -9c > "$TMP_DISC_DIR/initrd.gz"
 # build iso
 cd "$TMP_DISC_DIR"
 rm -r '[BOOT]'
-"$BIN_XORRISO" -as mkisofs -r -V "ubuntu_1804_netboot_unattended" -J -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -input-charset utf-8 -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin -eltorito-alt-boot -e boot/grub/efi.img -no-emul-boot -isohybrid-gpt-basdat -o "$TARGET_ISO" ./
+"$BIN_XORRISO" -as mkisofs -r -V "ubuntu_1804_netboot_unattended" -J -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -input-charset utf-8 -isohybrid-mbr "$HYBRID" -eltorito-alt-boot -e boot/grub/efi.img -no-emul-boot -isohybrid-gpt-basdat -o "$TARGET_ISO" ./
 
 # go back to initial directory
 cd "$CURRENT_DIR"