It is possible migrate all your ads from your existing website. We have migrated one site previously from x-zero 4.95 to classibase. All ads, categories, custom fields, users (with new passwords) transferred without problem.
Detailed info about migrating existing site to Classibase can be found here http://classibase.com/how-to-import-ads-from-other-classifieds-website/
Old URLs cannot be transferred due to different structure in scripts. In our past work we transferred site with 3K ads which was getting 7,5K visitors per month. After switch visitors dropped to 5,4K then after 5 month recovered, after 12 month tripled reaching 23,7K visitors. Also this may depend for your niche and other dynamics of internet. All non existing URLs from old website will be redirected to home page.
Regarding installation it is very easy and we can provide assistance if you have difficulties installing for free.
For migrating your existing ads to Classibase it will be free only if you purchase “Forever plan” license. If you want to purchase 1 year license then we will ask 100$ for migration on top of 1 year license price.
Also you can migrate it on your own, just follow instructions posted in above link. We can verify your updated xml file via url after you update it with your server data and image url details. Then you can use that xml url safely in ClassiBase ad importer tool.